dry hopping time depends on a few factors such as the amount of hops you are using and how you are dry hopping(pellets, whole flowers, hops floating on top of beer, hops submerged, ect.) it is safe to dry hop for 10-12 days with generally 1.5-2 ozs of hops. depending on what style beer you are brewing i would use multiple hop strains instead of one to get a fuller flavor, but this is only from my experience. if you dry hop too long some say you may get a grassy flavor although i have never experienced this. also from my experience the higher the abv the more you should dry hop. someone correct me if im wrong about anything i have said, but this has always given me great results on ipas, pale ales, stouts, amber, and red ales. also people will tell you to dry hop with low alpha but high alphas are also great for dry hopping, my number one go to dry hop is centennial.