Stian Gudmundsen Høiland:
When you are stably on the object of concentration, what is the spatial quality of that? How tight/narrow is the attention? . . . It felt as tight as when you stare at the tip of your nose for some while, but without the eye strain and headache that would follow staring like that.
Not quite sure what you mean by "spacial quality" with regard to concentration unless you are using an actual object at some distance from you (or mentally) to focus on. I use the breath, so I'll go with the other references you make.
First, lets make sure we're on the same page with regard to "access concentration." You realize that the term "access concentration" is a translation of the Pali term
upacara-samadhi, and in the exegetical tradition is marked by the suppression of the five hindrances, the manifestation of the jhana factors, and the appearance of a luminous mental replica of the meditation object called the counterpart sign (
patibhaga-nimitta). Although I don't necessarily always go by the exegetical tradition, I sometimes use it as a loose guide. In the present case, it might be helpful. Now, that so-called "counterpart sign" could be anything, either visual in nature or sensual (by touch) in nature, like a bodily sensation. If you use an actual object in nature, such as a kasina disk or some such, that's fine. I use the breath, to which I find the sensation of pressure in the center of the forehead to be the equivalent of the counterpart sign. When that sensation arises, I know the mind is concentrated firmly on the breath. The same, for you, might be true using a visual object as a counterpart sign.
In attempting to describe these processes, I personally endeavor to keep things as simple as possible, something that the ordinary reader can readily relate to. I don't differentiate between the three levels of
samadhi — preliminary concentration (
parikamma-samadhi), access concentration (
upacara-samadhi), and absorption concentration (
appana-samadhi) — as many instructors do; I simply refer to it as "establishing concentration" (or
samadhi). In other words, concentration is concentration is concentration. Yes, I recognize the subtle differences pointed out in the three levels mentioned. But the way this is described in the discourses, the term concentration (or
samadhi) is usually the only term used, since concentration is concentration is concentration. It would seem that Gotama wanted to keep things simple also.
If you are able, as suggested above, to "stare at the tip of your nose for some time," without unnoticed interruption or distraction for at least two minutes, then you have acquired a sufficient amount of concentration for further exploration of your practice.
Stian Gudmundsen Høiland:
And also, how long would you say that you have to stay stable in this manner for the appropriate nimitta to arise?
I ask this because I managed after about 2 hours of concentrating to "sink in" and stay exclusively with the "anapana spot" for 10 straight seconds. Then I overthought it, fell out of it, managed to get back in again for 3 seconds, overthought it, back for 2 seconds and that was it. I also seemed to perceive a "splash" of light in the center of my perception, and I think I could choose to look at eithter that or the anapana spot, even though they were in the same place. This spalsh of light was also very unstable and I couldn't focus on it without it "fading out" as I did.
The time might vary. With some, it may only be a few seconds. With others it may take several minutes. The temporal dimension just depends upon how quickly the mind is able to establish concentration on the object before the arising of the nimitta. For example, with the breath as object, it can often take but a few seconds of concentration on the breath for me to experience the counterpart sign sensation. If the mind is particularly active and unsettled, it may take a little longer. Generally speaking these days, I can usually acquire the counterpart sign rather quickly within a few seconds to a minute or two, shortly after beginning concentration on the breath.
I hope that answers the question you were asking.
In peace,
Ian