How much money to tip professional movers?

We're moving from a 920 sq ft place to a 1600 sq ft place both one story and only 15 minutes away from each other if any of that matters. Last time we moved from the 2nd floor of a building and it took the movers 3 hours to move us so we suspect it'll take less time since there are no stairs. There will be 2 of them so how much do we give each of them for a tip? The weather is crummy out too we are from Michigan .