Hi!
Your question is probably more appropriate in the
General Questions for Beatmaps forum there.
As for answering your questions...
1. It actually varies quite a bit per person! Someone can simply map as what they feel natural and go along, some people whittle it down little by little, some people spend a lot of time trying to think of ideas, patterns, and making everything perfect... People have lots of different approaches. It also depends on how much time you want to devote. Maybe it can take a few days to a week? Your mileage may vary, but it's obviously going to end up longer if you don't map much at all.
2. You should first try timing your map by trying to follow and create the beat, then let osu! calculate a starting BPM and offset for you. That's where you would press that button on the top-right or you'd press the T key to let osu! think what the BPM and offset of the song is. The results are often on the right track, but not exactly correct. This is where you need to adjust the BPM to something that's correct (usually it's the nearest whole number since most songs are constructed that way), and then you need to adjust the offset. The offset is like your Universal Offset in Options -- it moves the game's timeline backwards or forwards to sync up with the song, so when you place notes at certain intervals, it hits that corresponding beat of the song just right.
It pretty much is an experiment of trial-and-error. Listen to the beat to see if they match up, and try playing the map yourself and see what timing inconsistencies you have. If you notice your hit error consistently falls really early, for example, then you need to push your offset earlier to match up to that.
Good luck! You should check out
this video about timing for more info.