What is the avg life expectancy of an outdoor cat if he/she is well fed and in an area away from busy roads?

I inherited a cat from some neighbors a few years ago. I am not sure of the breed (she is long haired, possibly has some Maine Coon in her). She lives almost exclusively outside although I feed her at least 3 times a day (canned food) and bring her inside when the weather gets too cold (otherwise she stays on my front porch). According to vet records, she turns 17 years old next week and appears quite healthy esp for a cat of her age.