Thats a good question. Fortunately for me, my pins start and stop glowing right around legal shooting time, maybe a little fast, maybe a little slow, but pretty darn close. I had a shot on a 6 pt last year getting pretty close to the end of shooting hours, I drew back but didn't take the shot because my pin was too dark and the peep didn't allow enough light through.

So to answer the question, I agree with GMMAT, shoot when you can see the pins.

Ken