I did 11 days in the Tetons/YNP and felt satisfied, but have always dreamed of going back. I think it was 2 nights/2.5 days in Jackson, the rest working through Yellowstone. I had one day booked as a "swing day" for weather, etc -- I spent it in a leather chair on an interior balcony in the Old Faithful Lodge writing postcards while it rained. Very nice.

A few details. Get a map of yellowstone's roads. You'll see it's sort of a square sitting on a hexagram -- each "corner" is a major site. Figure about 30 minutes to drive each leg - it took me 20-30 on average, and thats in late September. It will take you longer. Add time for the inevitable stops to pull over and look at wildlife. The point is: if you're staying at Canyon, it's a 1.5 hour drive to Old Faithful, and a 1 hour drive to Mammoth.

I ate the bison burger at the Irma Hotel, with the giant famous cherry wood bar (was it really donated by Queen Victoria?). It was a bit gamey. In my 11 days, it was worth it (I actually drove the Bear Tooth highway to Red Lodge, spent the night, and swung through Cody on the way back in) Unless you're really, really into the rodeo, trade that for another whole day in the park. The views of a herd of bison on the northeast entry (my exit) road are one of the things that I pray I see next time I'm there.

Yellowstone is a big place. Depending on what you like, each polygon corner has something interesting, and each might take a day. Maybe you'll breeze the paint pots or something. Don't just sit at Old faithful. Take the walk north through the geyser basin to the great pools.