You'd be as good to go 12-12 from seed but you still wouldnt have a mature plant, a few weeks extra really isnt that long to wait to have some decent ganja. Have a look at the autoflower plants if you want to harvest in 8 weeks but even they might take a bit longer!
Autoflowers...very disappointed in them unless those buds bulk up in the next week to week and a half which is my harvest window. I'd just get a strain that flowers in 8 weeks. Some may take a little longer but you'll want to let the buds reach their full maturity and to reach their peak THC levels.