three weeks is problably ok, depending on fermentation technique. It's good to know your OG, what strain of yeast did you use? Knowing the strain and how much it will attenuate will help you predict what your FG should be. Waiting longer won't really hurt, especially since it's a stout, and not an IPA where hop character is going to start to fade. Some people say 1 week in primary, 2 in secondary, and 3 in the bottle. Since I don't bother with a secondary I usually leave most beers in for about 4 to 6.