Life expectancy is based on many factors:
Breed
Overall Health (History of Feline HIV, urinary infections, overweight, etc.)
Where it lives (indoor, outdoor, both)
What you feed it
I had three cats that lived indoors, only going out periodically:
1 purebred Maine Coone
1 stray white and overwieight
1 stray mix breed calico/himalayan
The white cat died after 11 years (cancerous mouth ulcer)
The Maine Coone died after 15 years (stroke)
The calico 'disappeared' after 18 years (she went outside one day, never came back. She had a major thyroid problem. Most likely left to die on her own. Cats do this.)
So as you can see, there was no set life expectancy for these cats. My wife's mom had a stray that lived for 21 years. It was completely indoors, pampered to death, and ate very nutricious food.
I can tell you that siamese have a history of short life spans. They're also known to go blind as they get older. Yours being a mixed, may not be a factor. I'd say just enjoy your cat with the time it has left. Feed it food based on it's health and age. Get the Science Diet, IAMS, or other high-end brand that's geared towards older/adult cats. Don't let it drink milk. It's a myth that it's good for them. Take it to the vet at least once a year for a checkup. Doing this, your cat could easily be around for another 5 - 8 years.
