As soon as they flower (show white hairs) you should switch. Conditions are different everywhere and every year and some strains flower earlier than others. Right now I have three different srains; one is in full bloom and two are bigger but not blooming yet (as of Aug 10th in Oregon).
I fertilize at half strenght every time I water and I water every three days - I have 20 gallon pots with good drainage. Using fish fetilizer which is very mild at that dose.