The official site of the museum shows the following opening times:

Museum in Tarquinia: Tuesday-Sunday 08.30-19.30 Necropolis of Monterozzi Tuesday-Sunday 8.30-sunset (you don't want to walk around in the darkness there, not because of the tombs but because of the rough paths)

Combined ticket 6.50 euro, ticket offices close 1 hour before.

I remember it as a short drive from the city center so I guess you could walk there, but maybe you'd prefer to take a cab or a city bus, considering also the train station of Tarquinia is about 3 km west of the town and the necropolis is 4 more km in the opposite direction. Hope this helps. Bye

Paolo Trieste, Italy