I would say about 1 1/2 - 2 years. If its the ONLY fish in there.

"Papa Red" wrote in message
...
Just how quickly would an Oscar that is currently only about 2½ to 3
inches in size, out grow a 30 gallon aquarium? I am thinking about
purchasing one, and placing him or her into an aquarium that is
currently equipped with both a Dolphin 200 and a Whisper power filter
20, to help keep it clean. I might also add a Pleco to the tank to help
keep it clean too,...Any help on this matter would be very much
appreciated. ~Dean.