I change the batteries every Spring, and today changed the AFM and FH, both got new Duracells last Spring, expiry date 2014.
AFM was fine, but FH had quite a bit of corrosion. Didn't look too bad yet, although a fair build up on the IC directly below. Once it was cleaned and back together it booted up, but no response to the test etc switches in the coin door (nor registering coins). Looks like that IC was taken out (it controls that area). This leaking must be less than a few months old as I had the backglass out with El Timbo over here and would have noticed the build up as it was obvious.
As you see, the "once a year" change rule won't save you every time, but it did hopefully confine the damage on this game to just an LM339 chip I hope.