Yellowstone campground lengths?

I live in Cody. Since you're planning in entering from the east, one nice place to stay is a small National Forest campground called Hunter Peak where you can make a reservation - $15/night (at least you could last year) - go to recreation.gov. That would make it convenient to enter through the northeast entrance and begin with the Lamar Valley area which is my favorite part of Yellowstone. It's about a 20 minute drive into the park. There are a couple of reservable National Forest Campgrounds outside the east entrance - Rex Hale, Three Mile and Wapiti. Three Mile doesn't open until July 1. There are several other National Forest Campgrounds that usually have available sites during the week. Ten miles west of Cody, you have the Buffalo Bill State Park - at the camping area at the west end of the reservoir, there are electric hookups and shower facilities. There are lots of options. For most of these National Forest campgrounds, bear spray is recommended.