If you are traveling a good distance to get there, plan on spending the better part of an entire day looking at the museum's exhibits. There are some areas where you can sit outside on the lawn to enjoy a picnic lunch, so plan for that. There aren't too many places to grab a quick meal out there.
You also should spend some time (couple of hours??) across the street at the Strasburg Railroad (take the tour of the repair shop), and perhaps another hour or two in the model shops just down the road.
Enjoy the trip. This is one of the best museums I've seen in a very long time. Let's hope Pennsylvania keeps up the funding for it!!!