Don't want to sound insulting, but I'd change the batteries in both FOBs whenever one of them goes down. When one goes down, the other isn't far behind.
I always carry a spare in the old map pocket of the pizza box. It is so annoying to have that darned alarm blaring and the lights flashing while you're trying to remember the correct turn signal button combo to punch in a pin.
Another thing I found out...if you have fresh batteries in the FOB, the range will be increased so you can fire up the bike in the attached garage with the keys and fob sitting on the kitchen counter and drive to work. Of course when you get there and shut it off, then try to start it up again, the fob still sitting at home, the security will do its thing and the bike won't start. Then you fumble around trying to find that wallet card or slip of paper with the manual bypass procedure and pin on it while the siren is wailing and the lights are flashing.