Hi Gordon,
Can't give you an exact answer to your question, but I recharge all three after three working days which I think is pretty good. To date, none of the backpacks have actually died on me. Mind you, I have a van-mount, and of course 3 of the said units.
One tip: don't let them run COMPLETELY dry. Sucks air into the unit which'll take a minute or so to clear.
Buy one.... ..or three!
Gary