It depends on the strain of plants. Some strains(photoperiod) can flower in as little as 7weeks, while others can go as long as 16 weeks. Most of the hybrid strains will flower between 9-11 weeks. I would say if you don't know the strain of your plants then fflower them to 7weeks and if the buds are still producing a load of white pistils then keep going. Once the buds appear to be stopping their production of white pistil hairs, then its time to start looking at the trichomes. When they go from looking clear and glassy, to mostly milky white, then you are coming into the harvest window.
Then its a matter of how ripe you want to let them get. On typical hybrid plants, people who grow their own will let them ripen until they see all milky trichomes with anywhere from 5% amber trichomes to 50% amber trichomes.
Do you know what strain you are growing? The breeders always have flowering time estimates. Depending on whether it is a fast flowering indica or a long flowering sativa can make a huge difference. Shorter flowering strains can take as little as 7 weeks. Long flowering strains can take up to 14 weeks.