I was very deep though, completely absorbed in it.
(...)
Questions: How intense is the "bliss" of deep first jhana supposed to be? I realise that it's not a quantitative science, but hopefully there is some consensus that can guide me.
Well, what are the benefits and wastes in continuing to practice this mental pleasure?
1) The mental pleasure is free of external object so the concentration in our mental pleasure -- just jhana, just pleasure in the breath, for example -- can help train a mind off of addictions. But regular good exercise, diet and group therapy/sharing/support can do that, too.
2) This practice of an innocent source of mental pleasure can help an anxious brain become calm and happy --- But regular good exercise, diet and group therapy/sharing/support can do that, too, and probably faster than learning to meditate.
3)So the actual use of the jhana of pleasure (which is second jhana) is to train the body in a friendly way to come to equanimity (fourth jhana).
The "waste" in continuing too long the practice of mental pleasure on the breath, in buddhist terms, would be that the practitioner wastes their chance to understand things as they are because they have become addicted to the sensations of the first three concentrated mental states (jhanas 1-3) and are not considering fourth jhana, equanimity, which involves just sitting but just sitting with well trained, alert attention and a trained, often at-ease body (often after much unease).
4)So you could keep studying the absorptive pleasure and I'd say, yes, study that if it is compelling to you (else it may nag you and 4th jhana will not arise when there is any desire present, in my limited experience). See what happens when you augment, move around the energy. Study closely what you do to recover the bliss when the feeling starts to subside, study what you feel and emote when the bliss starts to subside and you no longer want to sustain it, study how you feel after this meditation is completely exited, ask "Is this absorptive pleasure useful in my daily life, a) how and/or how is it insufficiently satisfying in my daily life?
5) To be clear jhana training is a friendly way to move the mind into choosing for itself total equanimity. One could do this through zen, "just sitting", open awareness and mantra/kasina practices, but anapanasati is a very simple four-jhana system to let the mind first taste the pleasure of steady, sustained focus (i.e, 1st jhana), to isolate and suffuse the mental feeling of pleasure (i.e., 2nd), to allow that pleasure to subside into comfort/stillness/mild pleasure-vibrancy (3rd, sukkha (sometimes people experience body boundaries ceasing here or in the lower jhana transitions --- feeling of just being a blob of pleasant, calm, still energy)) and then finally the mind must be so accustomed to these jhanas so un-wowed by them, so nearly bored by them, that it totally lets go and just sits, but sits with alert well-trained attention. Now fourth jhana attention - the attention of utter preferencelessness can arise, an very intimate and harmless form of attention. 4th jhana arises on its own, arise on the training; it cannot arise in the presence of desire, in my limited experience. So boredom and giving up are good affective precursor mental states, in a way : )
This practice and your mind are your lab, so it is totally your choice in how to spend its resources and I respect that each person has their urges to study this way and that way. What I've written above is just my take.
Each person has previous supporting conditions, so what I write here is not gospel, just what I know of myself.