You can overexpose, and you can underexpose.
Personally, I use a 300W bulb I found at Lowe's in an "exposure box" I built for 2 minutes. Experience and experimentation will give you a better idea of your own needs.
For a thorough test, make a circuit design that fills a PCB board, or just fill a similar space with a cross-hatching pattern (doesn't have to be a working circuit), and tape the pattern to the board. Use a black card (or other temporary light-blocking material) to cover most of the board. Leave about 1/2" uncovered. Expose this arrangement for about 10 seconds, then move the card another 1/2". Expose for another 10 seconds, then move the card again, etc. until you have exposed the entire PCB. When finished, the last strip will have been exposed for no more than 10 seconds, and the first one for around a minute, none of which may be enough (depending on your exposure rig), so expose the entire board (with the pattern still taped on) for a minute, to push the exposed strips' total time to somewhere around a 2 minute range. Develop this and see how different exposure times affect the resist layer in your exposure rig.
I have more trouble with the developer. That stuff's operating temperature seems terribly touchy. If it's too hot, it'll take everything off almost instantly, leaving you with a copperclad. If it's too cold, it seems to "shock" your resist layer, and the stuff never comes off. I found mine has a happy temperature at 115F. It's that Datak positive resist developer, in case you're using the same (it seems common).