I think you could probably begin just watering now. Soil has a good retention of nutrients in it already. As you water the little bit of run off every few days will just keep dropping the nutrient levels in the soil, allowing the plant to consume the residual nutrition and be as clean and pure as possible when you harvest. I agree that 3 weeks, 20 days more or less, is about how much longer that plant has. I could be wrong though, it could be more like 30 days, and in that case... I'd keep feeding for another week and a half.
Water for the last 20 days works well for a lot of people. Others will feed right up to the last week and then take the time to do a massive flush. Either way... You'll get some sticky, that's for sure. Fucking gorgeous plants. +Rep