Yeah I say about 2 weeks. I let mine go until I see very slight yellowing, or after the first set of alternating nodes.
Then I feed, and go once a week from then on.
Oh also, for those first 2 weeks when they are still babies i only use 1/2 strength nutes, then move up to full after a res change.. just an FYI to not burn your girlies
How long you guys let your DWC plants veg? In my past grows i've always used nutes from day one and it never seemed to hurt them and I always vegged for 21 days. This time around however, I am not using nutes just yet I wonder will there be a difference