Not a 2.5, but my 3RB's battery died after 27 months from new. Only extra item from stock that affected the battery was the (Subaru genuine) alarm.
95+% of the trips in the car were at least half an hour, so it's not like constant short trips killed it.
When it went, it went quickly. For two maybe three days at most, it took a second or two longer to start than normal. Then the morning it died, my missus took it down to the corner shop to get some bread, then drove back home. I hopped in 15 minutes later to take the kids to school, and it was completely dead.
I know a lot of people say you should only expect 2 -3 yrs from a battery, but my opinion is that only crap batteries will last such a short time.
I guess, therefore, I believe the battery in my 3RB was crap
Hmm, my 07 TBSTi will be hitting two years soon, might get proactive about it now....