I asked my vet this question 'cuz I can't even imagine one of mine going... She says her goal is 20 years but depending on the cat - 15 to 20 years, that is for inside cats though - outside cats 5-10 years.
The oldest cat that I have heard of was 30 years old. I have heard many people say that ther cats are betwenn 15 and 20. Mine are only kittens, but I wish for a very very long and healthy life for them.