I'd estimate that your normal run of the mill cat, barring any special health problems like cancer, Feline leukemia virus , will live 15+ years. I've personally never had a cat that didn't live to be older than that. None of my cats have ever had FeLV or cancer or anything though.
Never trust a tall dwarf... he's lying about something.
I had to have our family kitty put down just two weeks ago . He was old and sick. He could not keep weight on and his kidneys had started to shut down causing seizures. These problems started only a few months ago. He had just turned 18 in human years, which equates to about 70 and some odd cat years. Indoor cats, if they are kept healthy, will usually live at least 15 years, but many live longer than that. I known cats that were 20 years old! I think the oldest cat on record was 28? but I'm not positve.
Catfancy had an article about old cats a long while back... I think the oldest cat featured was 27 or 28. But they featured several cats over the age of 20. Not that this is an average or typical age but its good to know whatkind of commitment you may be getting into. If any of mine live that long, I'll still have them when I turn 50!