When they will be done depends on the natural light schedules where you live, AND the genetics of the strain. Without that info, the best guess I have is the middle of October, to the end of November... Again, all depends on genetics, and when they start flowering.
To get the most out of them: As a new grower, you want simple. Go online and look up Earth Juice Rainbow "pro grow" and "pro bloom" ... order yourself a 5 pound bag of each. Top feed the pro grow until they start to bud, then switch over to top feeding the pro bloom. Based on the size there, I would top feed about 4 tablespoons of the pro grow once a week till flower, then stop the pro grow and switch over to the pro bloom in the same amount.