What do you enjoy? I'm a firm believer that the best way to get good at time control X is to play a lot of time control X, and avoid time controls Y and Z. So try some different time controls and figure out which one you like. You can always change later.
And do lots of tactics problems.
30 min or longer. But the thing is in long games you should think more if you play a 30min game like a 15 or 5 min your ruining the whole idea of long games
If you aim at improving, then the longer, the better. If it's for enjoyment, then it's up to you.
2 hours
It is 30|30 and 45|45 for me. Play with increment, you don't want to timeout in an easy endgame after a great game.
15 minute, 30 minute or something shorter or longer?