Batteries carry multiple year warranties, meaning they're supposed to last years. Are you really concerned with a battery lasting for example 3 years vs. 3.5 years? Don't worry about it.
Only if you don't drive it. Driving it charges it, so there you go. The original battery in an E46 tends to last 6-7 years. All replacement batteried seem to last 5-6 years for some reason.
I picked up my M3 in December of 2002. I just replaced the original battery last week. I was amazed how long it lasted. I am hoping this new OEM last as long!
Read this if you are interested in a brief explanation of battery lifespan info... http://www.mpoweruk.com/leadacid.htm
