For those with guitars with active pickups, how do you know the battery is dying? How often do you change yours?

I've had a Gibson Explorer with active EMGs for 2 and a half years, and I've not had to change the battery yet. And it's not like I never play it. I know this doesn't answer you question but they do last a long time It is always a good idea to keep some spare batteries in your case though Just incase it should happen to die on you one day

LOss of output/no output. But yeah, NEVER more often than once a year and that was with massively frequent playing. Be sure to unplug guitar when no in use.