General rule, the closer the 13V your battery tests at the better (after sitting an hour or more). The closer the 12V (say 12.1v, 12.2v) they battery is dying and will put a strain on you DC to DC converter.
I would hazard to guess that most people who have a 12v battery over 5 years probably have their car garaged a lot, and/or plugged in most of the time or a mild climate. Those that have problems are those that have to have their car parked 8-10 hours a day without the benefit of being plugged in or without powering on.
The best time to test the battery would be at the end of a work day before you leave work. Test the voltage before you start the car.
Just my opinion.