I check my Pronto regularly to see if it is low on charge - especially if I am planning a download. If it's showing low, I charge it for about 30 minutes or so after which the charger unit starts flashing.

I am not sure if the flashing means it is charged, but it always seems to be fully charged at that point. I certainly wouldn't have thought leaving it on the charger for any length of time would do it any good.