Google the strain you are growing. Not hard to find that info. If it's a cross, you have a window.
If it's bagseed, when growth of new fresh white pistils slows, get out a microscope and look at trichs... Range for indicas starts at about 6 weeks....you have a range of finish times from there... through the crosses... all the way up to some crazy sats that take up to 4 MONTHS to finish.