The time of exposure varies depending upon the whether conditions. In the hot summer day in the bright sunlight 20 minutes are sufficient for the exposure in the bright sunlight to make the photo emulsion tougher, but in the winter day you need 40 minutes for the complete hardening of the emulsion.
In the slightly rainy day you need at least 2 hours exposure time for complete hardening of the emulsion. Watch this video tutorial for more comprehension of the exposure time. You can watch all the free videos tutorials on the topic of the Screen Printing on this website.