This is a question I get a lot. Any my answer is, That depends. Many things factor into the equation of how long to expose the screen for like, screen mesh, type and age of emulsion, type of lamps in the exposure unit, how old that lamps are, and the list goes on. So what to do? You need to do a step wedge test. A step wedge test is basically exposing the screen for a few different times and seeing which time produced the best image on wash out. There are a few ways to do this test. The expensive way is to buy a test strip for $30 bucks which is a piece of transparency that has a gradient of darker shades. The benefit of this test is you only have to expose once and the strip lets different levels of light hit the screen.
The cheap way to do a test wedge. Take construction paper and do 4-5 different exposures. Move the paper to expose more of the image each time. At the end you will have 4-5 different exposure times. Just pick the one that washes out the best and there ya go.