Prefatory Note: Whenever I enclose the word "jhana" in quotation marks in the comments that follow, please read it as referring to the "stock description of jhana" as this is found in the suttas.
Okay, Modus, I'm still attempting to better understand what you are asking. You said in the OP the following:
Modus Ponens:
I'm thinking of doing a period of intensive practice this summer, meditating between 6 and 8 hours a day. It will be no less than 30 days. What I need to know is if more time is necessary to experience all the first 4 jhanas and have some degree of control over them.
Which seems to indicate that you will not be studying during the summer, so no stress from that. But then you say below that the "reason I'm not thinking doing a proper retreat is because I'm under the stress of having to study; in the summer I have the opportunity to relax the pressure with my friends. A proper retreat could add to the stress of having to
do something. Thus, I would most likely fail in doing the proper retreat."
There's always going to be whatever "stress" you perceive there to be in attempting to dig deeper into the Dhamma and to realize the truth of the concepts that Gotama taught and to be able
to see those concepts being played out within the expanse of your own experience. That "pressure" (if you want to call it that) will
always be there. No getting away from it, in other words. But all this aside. . .
I applaud your desire to take it one step at a time, so let's start right there, and hopefully gain a better understanding of the achievements that you are endeavoring to accomplish. You state below:
Modus Ponens:
Regarding insight, I'm only interested in jhana at the moment. Only after attaining the jhanas I'm going to start doing insight.
In order to recognize "the stock description of jhana" (i.e. the first four levels as they are described in the discourses) as opposed to the more expansive definition of jhana which only includes the ability to "reflect, conceive and ponder over" (rather than having to characterize it as the 1st, 2nd, 3rd, and 4th jhana levels), one can become caught up in the perhaps misconception that achieving "jhana" (as in the stock description) is the
only way to achieve "jhana." Which it isn't. You can achieve a jhana state in which your ability to "reflect, conceive and ponder over" direct observations becomes heightened,
which has more to do with the increase in one's concentration ability than in the fact that they recognize the achievement of the 1st, 2nd, 3rd, and 4th jhana levels. The jhana state that I am referring to is very similar to
what I describe as the fourth level of samatha jhana. My point here is only to point this out. (This is perhaps an observation that only an advanced practitioner might appreciate and not one that a beginner might be able to relate to, so just keep that in mind.)
In other words, you may be able to achieve this state of jhana
without being able to identify having achieved 1st, 2nd, and 3rd jhanas. The mind just
goes there, for all intents and purposes unbidden and quite to the amazement of the practitioner. It can be somewhat confusing for the practitioner, since his conceptions have been
programmed to expect a recognition of the 1st, 2nd, and 3rd jhana levels before reaching the 4th. When this doesn't occur, it can be confusing to the practitioner. All I'm saying is, it
is possible to experience this kind of jhana state without being able to recognize
how you got there!
And while I can concur that achieving
this stock definition of "jhana" can be and
is very helpful in the effort to retrain the mind to be still and to follow the directions (intentions) given it without venturing off into distracted territory, it is also important to
realize that this isn't the only definition of "jhana" that is being discussed in the suttas, and that the ability to "reflect, conceive and ponder over" is just as important
if not more so than achieving the stock description definition. All this being said. . .
Even so,
it still takes insight into one's own practice in order to
recognize the experience of the stock description when it is occurring. So, to say that you will only become interested in performing "insight" AFTER you achieve "jhana" is a bit misspeaking if you understand that insight is not something substantial, but is a fluid process that occurs whenever it occurs, not only when WE
want it to occur or think it should occur. It's happening all the time. Saying this, I just don't want you to become too rigid in your thinking or perception about this issue.
The kind of "insight" that you most likely meant to refer to is the insight into the truth of the Dhamma which helps set one free, which needs a heightened level of concentration ability in order to maintain the conceptual picture of what is happening during the process of confirming the truth of what was taught. All I'm saying is that insight comes in all kinds and sizes of packages, big and small, and that it comes whenever it comes whether we are ready to recognize it when it arrives or not.
The question you originally asked ("What I need to know is[:] if more time is necessary to experience all the first 4 jhanas and have some degree of control over them.") has already been answered. As I stated before, it all depends upon the individual as to whether more or less time is necessary. You will have to make that determination YOURSELF (presumably), based on whatever insightful observations you are able to make at that time about your true condition.
The key factors here would seem to be, first, the ability to conceptualize just what the "stock description of jhana"
is so that one has an idea of (a good conception of, preferably with some similar previous experience that one can identify with and use as a conceptual model to pursue) what one is attempting to accomplish, and, second, following instruction that will inevitably lead to accomplishing that goal.
When I first began looking into achieving this "stock description" of jhana some dozen years ago, I first wanted to KNOW just what it was that I was being directed to achieve. From the reading I did at the time, I was able
to identify a similar experience to those being talked about by Leigh Brasington (in his online descriptions) and the hint given in the Mahasaccaka Sutta (MN 36). It took insight on my part to be able to make these connections, to be able to see what these descriptions were pointing at in terms of my own direct previous experience. Once I had a firm conceptual grasp on what this "jhana"
was and how to bring about the first, second, third, and fourth jhanas, I just had to trust in the instruction itself to help bring it about. Your mind will "go there" automatically if you are following something that you can experientially relate to.
This can be somewhat tricky to conceive when just reading about it. So, it is difficult to explain. You just have to apply some real world experience from your own life in order to accurately identify the experience being described. Or, if you are
mindful enough when attempting to follow the instruction, you will be able to recognize the arising factors (or at least their effect
after they have arisen) enough to be comfortable in confirming the validity of the "jhana" experience that you are having.
So, trying to put a time limitation on this process (as to when it will be achieved) is quite impossible. As I said, it differs from individual to individual. It all depends upon how quickly one realizes that "something significant" is occurring.
You should probably aim at just being able to
identify that "jhana" is occurring. Gaining better "control" over the process of their arising is something that develops with time. But four to six weeks (as you have indicated) is plenty of time to be able to experience "jhana," and to begin the process of exerting more control over this meditative state.
I hope that helps.
All the best,
Ian