Minimum stay at Yellowstone?

What is your opinion on the minimum length of stay at Yellowstone to see everything, or at least as much as possible?

Currently planning our last leave before we retire and thought about doing two weeks at Yellowstone. We spent four days there last year and that wasn't enough time. But we don't want to spend so much time that we spend a few days hanging around the RV bored. Especially when we can turn North and head for Glacier NP.

Any suggestions?