Hi Daniel,
I'm curious how I might expect my concentration to develop over time. Obviously I've noticed that on retreat my concentration shoots up and then drops pretty dramatically afterwards, but if I'm putting two or three hours a day into concentration practice will it actually pay off over time? Will it drop again as soon as I shift the balance over to more insight practice? If I practice concentration dilligently enough for long enough will I eventually reach retreat levels of concentration off retreat? I'm curious what others experience is in this area.
From my experience jhanic concentration, like playing a musical instrument, will develop over time and certainly does not require retreat. It just takes regular, sincere practice and it greatly helps to make it a joyful practice, finding an object of meditation that is readily welcomed by the mind which also causes no remorse or confusion or provocation. I use breathing meditation, because physiologically it calms the mind (and then sleepiness or boredom is the early training hazard). From my experience when the mind becomes very good at calming meditation and the body gets very still, insight happens on its own and "we" come out of the meditation with a new understanding of something or many "things".
So for me, the anapanasati instructions have been plenty: this method has mental stablilization, calming and insight all in one study. But it's not for everyone. Some people do need physical studies: Aikido, tai chi, music, cooking... so-called mindfullness training can also cause jhana, but this way can be harder due to the many, many movements of mind that are required to happen when one is awake and not simply sitting still.
Good luck with your study.