the common pleco's, (Pterygoplichthys sp) tend to be very quick growers when young (under good conditions) and can grow over 1" per month for the first 10", but usually slower in most tanks.
most other sp of pleco dont grow anywhere near as fast, most baryancistrus sp for example barely seem to grow throughout a year, so yeah, it totally depends on the fish in question, and also on the size of the tank, how much you feed, the tank temperature and also how clean you keep the water