sandj:
Osacrs are a truly wonderful fish to have for a pet if properly cared for. The other members have voiced their personal experiences and knowledge with you so you will become more familiar with them and your success rate with your Oscar will be long term.
The topic of "Why feeder gold fish or feeder fish in general, unless you, yourself raise them are not a good or even remotely good for your Oscar is because:
1] Feeders are prone to carrying internal parasites that causes a disease known as H.I.T.H. also known as "Hole In The Head" disease.
2]Feeder fish, may also cause other internally intestinal diseases in your Oscar, which are hard to diagnose, even by biologists or ichthyologists, unless they do a biopsy on the Oscar, which requires cutting the fish open to examine internal organs. This is a very long and expensive proceedure, known from personal experience.
Oscars do ingest some types of fish in the wild, which are small enough to fit into their mouths, but like you, they are selective in the fish they eat for food. Most of their's are indigenous to their natural habitat. Feeder's are not indigenous to their habitat and are tank raised by breeders of all sorts and you really don't know where they come from, thereby being a threat to your Oscar's health in general and in long term proper care for him.
I hope this helps you to understand WHY feeders are useless to Oscars. I hope this helps you to decide, DO NOT feed your Oscar, feeders of any sort.
Now, with that said, I wish to move on to you main question. "How long will it take for your Oscar to reach it's full size?"
Having raised a few Oscars, I have found each one is different. What I mean by being different, is that each one has a different character, personality, growth rate, eating habbits (the food they like or don't like), and temperment. Their rapport and disposition with you and other people (your friends) will be different. Most I have found to be kind and pleasant. But you also have those who are the extreme exception.
They usually attain 1 to 1.5 inches a month, based on their tank maintenance schedule which you must faithfully do to maintain optimum health and growth for your Oscar as well as proper feeding and interacting with them.
I had two very special Oscar which within 14 months, were monster sized (16" and 17" long from the front tip to tail end) for their age. Like most everyone else, I fed them a variety of foods. Frozen shrimp cubes, Hikari pellets, blanched (partially boiled) peas, zuchinni, brocolli, carrots, thawed frozen shrimp, blood worms, and their favorites: meal worms, nightcrawlers, may flies and crickets.
Taking into account they have voracious appetites, they will have as much bodily waste as they consume. It can be asthetically overbearing sometimes and on most days. I found it necessary to remove their waste every day. Thoroughly cleaning their tanks each week, and their filters twice a week, in addition to the daily cleanings of waste. I gave them a water 75% water change every 4 days. When their tank water levels dropped to, went below or was close to a 2 inch mark I put on their tanks, I would top it of by adding treated water, so as not to stress them out.
As far as tank size, a 90 gallon tank is suitable for most, but a 100 up to a 125 gallon with a footprint of 72" long and 24" to 36" from the front to back of a tank would be an ideal tank, a big bennefit for them and they will truly enjoy the room to move around.