I usually will replace my batteries when they hit 10 years no matter how well they are working. I once got a IIfx in that I had to send to the recycling facility since the batteries had leaked all over the board--and it was only about 12-13 years old at the time. (Of course, I did strip it of its hard drive first since it was a huge internal SCSI--hard to come by even in 2003).

The oldest working battery I have ever seen is 14 years (1989 manufacture date in 2003, installed in an SE).