If you want to just go through each battlefield and hit each spot for pictures then you could do Antietam in half a day and Gettysburg in a day. If you really want to study the ground and take it in then it will take longer.
I especially enjoy both but Antietam is my favorite because I can park at the visitors center and walk virtually the entire field, Burnsides bridge is quite a hike but the remainder is easily accessible by foot. My main criticism of Gettysburg is the amount of driving, not that big a deal, one has to do to access the different areas.
Luckily touring Antietam chronologically it's all downhill from the Hagerstown Pike to Burnsides bridge.
Gettysburg is a bit more pedal work. I like the ride down Sedgwick Avenue past the roundtops, many Union monuments, into empty fields and then the first CSA marker appears as the road becomes south confederate avenue and sloooooowly bends north onto Seminary ridge...