It depends. The NPS normally says 2 hours for the driving tour. To see it all simply can not be done in a couple of hours. The wilderness has many foot trails and off park locations.
The exhibit shelter off Route 20 has some neat information to read through. There is a ~2 mile trail starting and ending there that loops through the battle field with some informational stops along the way. The beginning is paved but the rest is dirt. It's not a difficult hike, depending on your pace, could be done in an hour or less. You can get maps of the driving tour at the Chancellorville Visitors Center on route 3.
I set aside nearly a full day, but if you just want a basic overview and stick to the Park's driving tour and associated signage it probably can be done a couple hours or less.
It is sprawling. A couple hours if you want to see it all.
To see everything on foot, I'd say at least 2 hours. It's worth taking your time, though, to fully appreciate the courage and sacrifice that occurred there.
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