what I find incredible is my AVP still had the original battery in it and it still worked up til last year... when I of course replaced it finally.
I suppose now is a perfect time to mention that if you buy the CPS2 batteries from Jammaboards.com, there have been reports of people receiving dead batteries.
I also know of a lot of people that install "keystone" battery holders or various other battery holders. these are kinda frowned upon because you introduce multiple failure points and if in the event you dropped the board, you would most likely pop the battery out somehow. there's nothing wrong with soldering 2 leads.. it's pretty painless, especially if you have a proper desoldering tool, even the Radio Shack one is perfect.
the approach that I've read (and have executed fine 3 times in total) is that you power the game on for 20-30 mins, power off, disassemble (be certain you own a security torx bit set, I don't remember the size off-hand unfortunately), have your soldering iron ready, and then make the battery swap, noting obviously of the polarity. (if you install the battery backwards you're screwed.) the reason you run it for awhile is that it keeps enough of a charge to enable you to switch out the old battery for a new one without the voltage dropping to 0.
after that, if you consistently play your game, you'll have a charge running through it to where I guess it won't rely on the backup battery so much.
so we've all seen the original batteries last like 15+ years.. how long should we go before replacing the replacements?