It's a straight run up Rte 15 to Gettysburg that would take about 90 minutes. I just got back from the new Visitors Center and can highly recommend the trip. The center has completely revamped the museum to create a very educational and entertaining history of the Civil War with, of course, a focus on the 1863 battle. The Cyclorama painting has been restored and now presented with a light and sound show. The visitor center can be done in about 2 hours.
If you want a guided tour of the battlefield you will need at least 3 hours. I would advise TDI members to reserve a guided tour in advance. The guide will ride in your car for $55; a per-person bargain compared to the $35 pp bus tour. You must reserve a tour guide at least 3 days in advance.
Remember that it is a holiday weekend and there will be crowds.
Another piece of advice: Be sure to buy your tickets to the museum online before leaving. There is usually no wait at the Will Call line but you may wait over 30 minutes in the other ticket lines.
http://www.gettysburgfoundation.org/visit/center.html