Depending on how often you use the buttons, it can last about 2-4 years. These batteries are dirt cheap to replace if you order them online.

Maybe you press buttons accidentally when the keys are in your pocket. It used to happen to me ALLLLLLLL the time

Just started year 5 with mine, but I've only put 12,700 miles on my '09 and probably used each of the two fobs intermittently.

Also, incidentally,
 

when you do replace your battery, go to Radio Shack, not Nissan. Nissan's battery, from the parts department, is way more expensive than the same darned thing from Radio Shack---and the kid at Radio Shack will put it in for you, too. I did that with my Altima coupe.