It really depends on what type of onions you are growing. Long day, short day, or day neutral types will all bulb up at different times. Poke around with your finger and feel for the bulb, and that will give yo a good idea of when they will be ready.

Kevin,
Like Landscraper says, it does depend on quite a few things.
For storage onions, generally if you see a full sized onion at the base they are getting close. Next, check the neck of the plant just above the onion bulb. When the onions are finishing up this part gets soft and the tops start to flop over. If it's still firm, you have some time yet. When it does soften, stop watering and plan on harvesting within a few weeks.

If you do a google search on "when to harvest onions" you'll get a ton more info and some pictures too.