How long will it run on a full charge?

Depends. 40v batteries come in multiple capacities, 2ah, 2.5ah, 4ah, and 6ah. It's a 20w light, so the smallest battery (2ah, which is 80wh) should last about 4 hours on high, and 8 hours on low. The 4ah would be ~double that, and the 6ah would be ~triple that, for potentially up to 24 hours on low with the 6ah battery.

Over four hour on bright and about nine on dim