When you bought the clones/seeds, wasn't there an estimate on the flowering period? I would guess around 8 weeks, if the pistils are turning already. Also, you might want to check for a hidden hermie, because the pistils will turn brown early if they get pollinated. I had that happen last run, but got lucky because only 2 plants got pollinated out of 8 of them. I still don't know how the hell that happened, because they were all packed together touching each other, with a fan blowing on them constantly. Hmm......I guess I was lucky? lol
Anyway...the only way to know for sure, is to keep an eye on the trichs and wait for them to start turning amber.(10%+) Then you'll know they are finished for sure.