I felt my first kick at 15 weeks
I felt my first good kick at around 20 weeks :)
I felt my first flutter at 19 weeks :)
Depends on if you've been pregnant before, where the placenta is located, how the baby is laying, etc etc. You should feel movement anywhere from 16-24 weeks. As for how long you can sleep on your stomach, for as long as it is comfortable. I slept on my belly until I was 35 weeks (I didn't get large at all while pregnant).