A day, and it’s awesome.
There was a cave I toured too, on my mission, but I can’t remember where, but Google shows they have a few caves with tours.
DMZ tours are fun, I didn’t get to go to Panmunjom, but I visited an out of the way spot north of Sokcho.
There are resorts with beaches, the surf is mild out there, etc.
You can burn 4hrs+ in the War Museum in Seoul which also acts as a history museum.
Lots of stuff.