Arches NP
While I realize that you can never see "everything" in a lifetime, what's a reasonable amount of time to see the Arches, we do take short, relatively easy to moderate hikes, but not "avid" hikers. but...
Forum Discussion
You can see Arches in half a day if you want to just get an overview and take a couple short hikes. But the Moab area has lots of other (better IMO) places to visit too that I would encourage you spend at least 3 to 4 days (minimum) in the area.
