In your opinion, how long a period of celibacy is needed to attain Jhana? Is it something you have to give up entirely? What do you base this length of time on?
Entirely? In my opinion - yes. Otherwise, as I said above, Buddha would never be so strict about that, establishing such strong rules and observances for monks. Though I had never been a monk, I
did practised a lot keeping only 5 precepts and having sex, and keeping 8 precepts (for periods from 1 to 6 months). And I saw for myself how it affects the mind - having sex and being free from it for some prolonged period of time. Sexual desire is the strongest of all sensual desires and it severly muddles the mind, so I can't see how someone "reaches jhana not abandoning sexual activity", these are just two opposite things.
Concerning the period - it depends on a personality. Someone who is not "strongly proned to lust" can make it easier and quicker. For another one it will take more time to cleanse the mind with numerous important preliminary practices, mentioned, for example, in MN 107 and many suttas like this. Most people, probably, will never reach jhana in this very life because of poor indriyas and strong defilements. In this point I do agree with Visuddhimagga, which says that a few practitioners out of many thousands of them will make it to jhana. I think jhana state is strongly underestimated these days in the sense, that this is something easily done, easily practised and does not appear something special, being just like some "slightly changed ordinary mind with some quantity of peace and internal pleasure".