Okay, I just purchased a pleco from my LFS. He is about 4 inches and I put him with my parrot and convict cichlids. He hides in my cave all day, and during the night he eats loads of algae. The whole back wall of my 29 gallon tank was covered in algae, now it is clear, and only a few spots still have algae. I know this pleco will outgrow my tank. How long will it take him to grow? The guy at the LFS told me they will outgrow my tank, but they grow slowly. I just need to confirm this. 

the guy in lfs is lying to you the pl*c will very quickly grow in a tank that is big enough for him to grow also depends on how much he eats you have prob got a common pl*c so you will be trading him in a few months cos he is too large and is rearranging the tank 4 you by this time you will have a strong bond with him as they are cool fish