how long should a natural gas furnace last?

this question pertains to the average useful life of a single family home natural gas forced-air furnace used for heating. the home is located in a mild climate (California) and thus the furnace is not used excessively.

” Typically a furnace or air conditioner will last 15-20 years.” or “… a gas furnace should last from 20 to 25 years.”

In your climate, I'd expect at least 25 years.