The average battery life in our FOBIKs (keys) is around 2 years. My 2011 was built 1/2011, bought it 11/2011 and the key I use all the time had a low battery starting 10/2012. Went to about 12/2012 and it still worked, but replaced it about that time. My spare one was dead sometime in 2013, new battery and it worked great.
It should be noted, if you are still under basic warranty, key batteries are covered under basic warranty. The above 2 times I paid nothing. I'm out of basic now, and I just replaced the battery in my primary key again after 2 years. Only a few bucks for a new battery.