7 miles last night uploaded. Watch has been sitting on the dining room table overnight, still not touched the charger and this morning the battery is showing 50%... Not being funny, but even if this is mis-reading and it all of a sudden dies a death, the battery life is incredible compared to any other Garmin so far.
39 miles/4.5 hours onto the 5th day of not touching the charger!
Kinda puts the estimate at around 10hours then which I think is correct for the 620.