It all boils down to weather you want to say you've been there or have you really seen them.
We did Glacier a few years ago for 4 nights and 3 days. While we saw a lot, there was still much that we missed. We have done Yellowstone many times over the years spending anywhere from 2 days to 3 weeks and have yet to see everything that we want to see. Yellowstone is huge and a short stay will usually make you want to come back for more.
We are leaving in a couple of weeks for yet another trip/pilgrimage to these wonderful parks. We will be spending 4 nights in east Glacier at St. Mary campground and then moving to the west side for 3 nights at a commercial park. After that we are heading for Yellowstone for 14 days at Fishing Bridge and then down to Grand Tetons for 7 days at Colter Bay.
Our experiences show that 5 days in Yellowstone is the bare minimum for seeing all the major attractions. There are many short easy hikes that will take up more time if you want. The Beartooth Hwy is a great drive with a motorcycle but not a RV.
Moving from campground to campground inside YNP is not productive. It just uses up more time tearing down, moving and setting back up. Pick a good centrally located campground and day trip from there. Fishing Bridge is the only one inside the park with hookups. Many people complain that it is tight and too close to the next campsite. Reality is that you won't be spending much time there.
Bottom line, spend as much time there as you can. Odds are you'll be back for another visit.
Keith
