You can wait as long as you want. Whether the prices will go up or down is anybody's guess.
You're gambling. Period. Can you afford the $275? If so, I would probably book. Yes, the prices might go down, but they might go up. It depends on whether you feel lucky. Once you book (unless on SW), don't look back if you'll get upset if prices drop.
Also, if you are booking with a legacy airline (ie: US Air, Delta, United, etc), don't be surprised if your flight times change between when you book and the actual departure. So just because you book an evening flight today doesn't mean you won't be changed to an afternoon flight in a couple months. There's no guarantee that will happen, but I don't want you to be surprised (and upset) if it does.