I would say an hour if you are buying tickets to one trail. Maybe more if you spring for tickets for a second trail. It is a 15 minute drive to the small crossroads of Poolesville where you can find some strip mall restaurants.
It would be nice if they had a Haunted Bus shuttle service from NW DC like the Calleva camp busses.
The other option is to go along to the haunted forest. It's pretty amazing.