only you can figure out the correct exposure time for your setup, your lighting, your temperature, your humidity, your thickness of stencil, and your emulsion, and a number of other factors.
ya gotta do the testing.
there will be many people that will tell you numbers. but the truth is, without knowing all of YOUR variabls, their numbers will mean nothing.
do yourself a HUGE favor, and buy an exposure calculator. their like 30-40 bucks, but they will last you a lifetime, and will save you hundreds or even thousands on wasted emulsion, time, and headache.
if you can't afford a calculator, do a step test. there are lots of videos on youtube showing the process.
this is the only way to know YOUR exposure time.