As a rough rule-of-thumb, 3-5 years, but it's like asking how long tires will last - it depends on a lot of things. Some people may get 2 years out of a battery; some seven years or more. In Florida, you can get by with a "weak" battery longer than you can in colder climes but the year-round Summer heat lessens the life span of the battery. If it's an "old style" battery, keeping the cells topped off will greatly extend its life.

I usually wait until I notice the engine spinning a little slower during startup before changing batteries.