Again, in what plastic? For Star and Champ, it's more like 2-3 years, not months. Maybe you play on really rough courses that tear your discs up. Even with a lot of use, I've yet to beat in a Champion plastic disc to any change in flight pattern.
I haven't found this to be true in my experience. Champion plastic usually breaks in pretty quickly(I don't mean to beat stage). But once it gets to that slightly broken in stage, it WILL tend to stay there for a very long time, if not for ever. But, stable to over stable discs tend to lose some of that dumpy-ness and hard fade fairly quickly. Maybe if you play in a completely open course you might never see ANY change, but a few(dozen) tree hits should do the trick for most players.