Depends on a lot of factors, including how many passengers fly the route out of your airport, how many flights there are, and how many airlines are competing on the route.
If you are wanting a non-stop flight, you generally need to book a bit earlier.
Also, if you are wanting to fly to a resort on a Friday and fly out of a resort on Sunday, you need to book further in advance. If you are wanting to go to a resort during a holiday period, you need to book further in advance.
If you are flying out of a major airport on a Tuesday, Wed., or Saturday in the off-season, about 6-8 weeks is usually the best bet, otherwise, book as soon as you can, or anytime there is a big sale on your route.
BTW, if getting a bargain is important, never "pick out" a flight meaning to come back later to buy it. If you are going to look at airfares at all, make sure you have your cc handy and the ability to check schedules immediately.