that will depend on your lighting and and if you have any stresses or not, but when your nodes start to alternate, your plant is muture, and it should show pre-flowers. This usually happens around 5-6 weeks or so, may be a bit more. if you can not wait that long you can take a clone when it is big enough and flower it. If your clone is female then so is the plant.
Hope this helped a bit.
Peace
Massproducer