I think staying away from commercial work has actually been an important reason that I have avoided burnout. My attention span is usually limited per figure to about 7-10 days, and I never seem to run out of things I want to do. I think if I had routinely got bogged down in 7-8 week master sculpture and engineering projects, haggling with mfrs over design and subject matter, etc., it all would have started to seem too much like hard work.
My love of golf definitely results in figure burnout for short periods during the summer months!
I did a few jobs for the Franklin Mint in the late 1990's. They paid well, but they kept me away from painting so long that my eyesight deteriorated, requiring me to wear glasses. The projects seemed to drag on forever (8-12 weeks each), and ultimately were never released by the Mint. One of the two - Buffalo Bill on horseback chasing down a very large buffalo was one I was quite proud of. For me, giving up control often means giving up the fun as well.
I got into this for fun, and keeping that sense of fun and pleasure is the key to longevity. If making masters is fun (and it is to many), by all means do it. If it isn't, avoid it and stick to what you enjoy.
Bill