The senate is sworn in on Jan 3, so the damage Trump can do (with partners in crime) is until then. His executive orders and such can continue up to when Biden is sworn in.

I expect a lot of flouncing around in anger, a total refusal to even pretend to be productive, and possibly Trump "pardoning" himself and leaving the White House to Pence. I predict that Trump will never concede, but I sure as hell hope that Twitter closes his account and doesn't let him have a new one.