1 or maximum 2 weeks. The easiest way to tell is: 1. when al least 75% of all pistils have turned red/brown/copper. 2. When the leaves start to yellow and loose their vigorous look. Don't be afraid of that, it's natural since the plant is using all it's reserves in order to grow those buds.
It's best to stop giving nutes in this period and give the plant just plain water in order to flush all remaining nutes so they dont f*ck up the plant's taste and aroma.
Hope that helped.