There have been several studies and articles I read suggesting that dry hopping over 3 days has deminishing returns and over 7 adds nothing good. For hoppy beers (I assume that is the goal, dry hopping and all), I believe the general consensus is to finish fermentation quickly with a strong pitch, dry hop during or right at the end of fermentation and get it in the keg for enjoying as fresh as possible, even doing the dry hopping in the keg sometimes when preferable. I don't think 21 days should be necessary having the beer sit on the cake with hop matter. Start dry hopping when fermentation is 80% complete, finish the dry hopping schedule no longer than 7 days, crash and fine 2 days, keg and carb and enjoy as fresh as possible. But I'm new to this as well and there's more than one way to skin a cat so it's all up to you and you can find what works best your way.