Skip the bags. Go with 12 hours darkness. I have played around with cycles greater then 24 hours. (16 on 12 off) and haven't noticed any advantage. The timers are hard to find and it is just a hassle. I'm not sure where you heard 6 hours darkness, (and I assume 18 hours daylight), but indicas and sativas will not produce flowers with that photoperiod.

Good luck