At Methodist, we get 16 days first year, 21 days every year after that, and weekend days don't count. It's PTO, so sick and vacation in one pool. It rolls over and they'll pay you back for the balance when you leave, up to one years worth (21 days).
In general, its first come first served and you ask as early as possible, but in general at least the month before you need off, so the schedule can be arranged. If you're on call, you find someone to swap with. If an emergency comes up, we have enough PAs and people we can make things work, no worries.
I know some of the CP rotations get antsy if you take too much time off because they only get you for a month or two to begin with, and in SP you have to make sure service duties are covered. Rule of thumb is the longer the absense the earlier it needs to be planned for.
All in all, I think we have a good system.