Most athletes know that their strength programs should have different phases, including hypertrophy, strength and power. Hypertrophy is a big one for athletes who are looking to build muscle (learn how to gain muscle faster). However, you can't get caught up focusing on this one phase alone.
So how long can you stay in this muscle-building phase? It's best to hypertrophy for 20 to 40 percent of your off-season. Where you fall in this range depends on your sport and the length of your off-season.
If you are a football player who needs to put on muscle, you might want to spend more time in the hypertrophy stage, since this is such an important part of the sport. If you are a soccer or tennis player who relies more on muscular endurance than size, the 20 percent range is more suitable.
Length of the Off-Season
Every sport has a different off-season. In general, a program switches to the next phase after four to 12 weeks. If you have the luxury of a long off-season and want to build significant muscle, you can stay on the higher end of the range. However, going any longer than this won't do you any good. Your body will have adapted to hypertrophy training, and you won't make any significant gains until you change your program.
Check out this link for a complete 36-week off-season workout program with an eight-week hypertrophy phase.