I have kept bass in excess of 20lbs. (yes, 20lbs) as pets. Growth in big fish varies so much I don't think there is a way to gauge it. My grandfather kept a 17lber for 7 years before the angler removed the fish and took her to a boat show...when returned she stopped eating (stress)
I keep the fish in my aquarium for 6 months now, used to keep them until death. There food comes seined from the lake, filled with water from the lake and they are eventually released back into the lake. Seen growth rates from fish caught in the summer pull over 5lbs in a year and I have actually had fish that ate like an ox that never gained more than a half pound.
http://www.youtube.com/watch?v=0oYht3k7pxA She is swimming back in lake fork 3lbs heavier (almost 13lbs) when brought in for an air bladder in October of 2009.
Edith was caught on lake fork March 22, 2002 as a 16.12, was put into the Texas Sharelunker Program and given to us approx 1 year later at 18.69...the angler transfered the fish to some exotic game dealer 18 months later and she weighed an outstanding 20.51 at 11 years old. Given some more time I fully expect I could have raised a world record.