Now how long should sew in weave really stay in?

I have heard so many things that if you leave ur sew in weave for more then 3 months your hair is going to get matted underneath the sew in weave and you'll have to shave your head and i sure as heck don't want that happen! i just really want to know whats how long should u really leave your sew in weave in so u don't have to go bald! D: