15-20 years. 20-30+ years if well cared for. For a cat to die younger than 15 is unusual unless they are poorly cared for. Quality food and fresh water are the best way to ensure health. As cats age, incorporate raw or canned food into their diet to prevent kidney/liver problems (dry food is hard to process, it requires a lot of moisture from the body, and as a cat ages, it becomes harder and harder on their organs to process dry food. Switching to at least a partial raw or canned diet gives them more moisture with their food and helps alleviate the problem. You can also soak dry food in water or chicken broth before feeding.)
Also, lay off the vaccines, especially in older cats- after kitten shots, cats really don't need anything updated except rabies. Even if you do update when they're younger on things like distemper, once they hit around 10 years old, don't do it. Indoor cats just don't need the constant vaccines and it really wears down their immune systems after a while.
Teeth are a big thing too- bad teeth cause infections in the gums that get into the bloodstream and cause organ failure and early death. Make sure you're keeping their teeth clean, again, especially as they age.
Lastly, keep the weight off! Healthy cats actually look a tad thin to most people. Fat cats are prone to a huge number of life-shortening health conditions. It's actually healthier for a cat to be slightly underweight than slightly overweight. If your cat gains weight as it ages, cut back on the food or switch to low-fat.
I currently have a cat that's 11 1/2 years, and the vet says she'd never believe his age because he looks and acts so good. He gets rabies every 3 years and eats Wellness dry and canned food. I have a water fountain to help encourage drinking (many housecats don't drink nearly enough water, which again, affects their kidneys). Because he eats quality food he has no issues with dental disease.