Well, I don't really have a set time when I stop giving N. I just use veg nutes during veg and then switch to bloom nutes when flipping to 12/12. Makes it easy. It has been my experience that yellowing due to N deficiency starts around 5-6 week of flowering and continues to the end (indica dominant), so yours does sound a little early.

My bloom nutes do have some N in it so I don't think you should cut N out completely during flowering.