Exposing a screen printing screen is like exposing a photograph. For the best results, you need to expose it for the optimum amount of time. If you expect it a little less, or a little more, you will still probably be okay. But the farther you get away from the optimum time, the more results will suffer. The variables for figuring out the correct time are brightness of the light source, and distance from the screen.
You need to decide on a light source. I currently use a 250 watt halogen work light with the UV filter glass, and safety cage removed (you assume all risks if you remove your glass and / or cage from your halogen light). In the past, I have used a 75 watt incandescent bulb in a reflector with a clamp. Whatever you use, make sure you have some way of clamping it, or mounting it above the frame for an extended period of time.
You need to place the light far enough away from the screen, so that the whole screen is being evenly lighted. If the center of the screen is significantly brighter then the corners, than the light is too close. The bigger the screen, the farther you are likely to have to place the light source. The farther away the light source is, the longer the exposure time will be. Measure the distances, and record it, so you will be able to reproduce the results in the future.
Now you are ready to run a test to find the best exposure time. Here are some base times to start with (based on a 150 clear incandescent bulb):
12 inches above the screen, 45 minutes
15 inches above the screen, 74 minutes
17 inches above the screen, 92 minutes
The higher the bulb is above the screen, the longer the exposure needs to be. If the wattage of the bulb is lower, you need to lengthen the exposure time. If the wattage is higher, then you need to shorten the exposure time. If you use a different kind of bulb, the time may vary.
Here is how to test for the best exposure time:
Coat a screen with photo emulsion the usual way. Print a transparency with numbers from 10 to 90 in increments of 5. If you are using a low wattage bulb, you should use higher numbers in increments of 10. With the light set up, place the covered screen under the light to be exposed. Use a clock or stop watch to keep track of the time. After 5 minutes, cover the 5 with an opaque piece of paper or cardboard. After 10 minutes, cover the 10. Do this until all of the numbers are covered. Then wash out the screen. The lowest numbers that are distinct on the screen will give you the optimum exposure time.