I just run a calibration test on my APC Smart 1500 UPS, it started at 09:33 and completed at 11:13. I'm not sure at what battery level it stops the calibration test (looking at the logs it was at 24% at 11:17) but it lasted almost 1.75 hours, load was 0.6amps which is 144w which is about right for what I have on. So it seems my 5 year old batteries are still in pretty good condition - I purchased it on the 28 Feb 2011