What type of emulsion or capillary film are you using?
What type of light source does your exposure unit have?
If you are using a commercially made exposure unit, the manufacturer will typically provide some guidelines for screen exposure times. If you are using "home-made" exposure unit, you will be on your own. Your best bet is read over the documentation from the manufacturer of the emulsion or capillary film that you are using. This will put you in the ball park. A exposure time calculator is available from many screen printing supply vendors but they have a limited usefulness as they are dependent on the type of emulsion that you use. Do some testing by burning a series of test exposures to determine the optimum time for your particular exposure set-up. If the stencil "blows away" during the washout, you're screen is underexposed. If it is very difficult (or impossible) to wash out the stencil, you're overexposed. The ideal exposure time is somewhere in between. Keep some notes in a little book of your exposure tests and refer to it for future reference as needed.