I don't worry about it.

When the performance becomes unacceptable, I just throw it away (uh ... 'dispose of it in an ecologicaly-friendly fashion') and get another device or another battery. Life is too short to fret over something like battery life, unless it's your profession (or, in lensman's case, obsession).

I just wanted some guidelines so I wouldn't do too much stupid and get a reasonable life out of these things.