We have completely upgraded our battery sets for all the detectors, we now use Sony batteries and the Sony chargers seem to charge a bit faster, these have pin and socket charger connections and make a better connection, the batteries are higher capacity than the old ones so i would expect 7 hours or so on a GPX maybe more, I have never flattened one out in the field with a GPX but I have with a GP.
We only received the GPX connectors yesterday, so batteries and chargers are now available.