It depends a great deal on what the party will do. If they are going to explore the castle first, it could take quite a while. Castle Ravenloft is a maze; it has plenty of combats, traps, secrets to uncover and the floor plan is horrible.
If they are going straight for killing Strahd, it would take maybe 4 sessions (4 hours each) to 6 sessions, depending on where Strahd can be confronted. Some locations are pretty obvious and easy to find even without knowing the details of the layout; other locations will be much tougher to find and/or makes the combat with Strahd difficult.
Note that taking Strahd down at a location other than the location indicated by Madame Eva is pretty tough; Strahd has plenty of ways to escape from combat quite easily, especially in his castle. Trying to beat Strahd that way isn't likely to work.