It depends on if you also plan on seeing nearby
Canyonlands NP
Dead Horse Point SP
Natural Bridges NM
Capital Reef NP
Zion NP
Bryce Canyon NP
Grand Staircase Escalante NP
If none of the above, then 3-4 days. I would at LEAST check out Canyonlands and Dead Horse because they are close by. Canyonland has some breathtaking viewpoints that aren't a long hike.
My favorite was Capital Reef, but thats because we ARE avid hikers. Great hiking there.
Also, if you can get there by driving on Rte 12, do it, and plan lots of extra time for pulling over to oooh and aaaah over the views.
Sue