Depends of allot of things. Normally you are looking at 60days+ once you start 12/12.
Some strains take longer some shorter.
Also the time it takes depends on the environment they are in too.
If you are using cfl's to flower they will take a bit longer then under hps.
Dont expect to much out of those little bulbs you wont get much bud using 2 25w bulbs for 2 plants.
The short of it is they are done when they are done.
Only way to tell that is get a magnifying glass or a jewelers loop. Got mine for $12 at my hydro store its a lighted 30x loop.
And watch the trichs when they go from clear to cloudy or cloudy to just turning amber it time to harvest depending on your preference.
If you want to learn more about how to tell if they are done check this out-->
https://www.rollitup.org/subcools-old-school-organics/66405-amber-alert.html
It will tell you all you need to know.