I'm assuming extract. If give yourself at least 3 hours, but preferably four. You don't want to rush. It takes a while for setup, it takes a long time to get to a boil (on a stove, anyway), it takes an hour for the boil, then a while for cool down, pitching, gravity reading, cleanup, etc. Four hours to be safe. And don't make any solid plans for immediately afterward. Good luck and happy brewing.