Your 15 year old window air conditioner might still work, but you might think about buying a new one with an E.E.R. (Energy Efficiency Rating) of 10.0 or better, because a 15 year old window air conditioner cannot be very energy efficient, and is probably wasting your money.
It can last forever. I have window air conditioner in my home from the last 20 years and it is still working very good. However, a lot depends on the maintenance. If you follow the steps to maintain it then you can enjoy it forever. To see, how you can maintain window air conditioner, visit this link.
Quite a while. 10, 20, 30 years even.. But I guess it's a good idea to change them every 10 years to keep up with energy efficiency technology.