My family is on our first big Tesla trip from Seattle to Big Sky/Yellowstone.
My supercharger experience (all during daytime as well) has been:
Ellensburg: 30 minutes - 1 other Tesla
Ritzville: 30 minutes - nobody
Coeur d'Alene - 1 hour - nobody
Superior - 20 minutes - 1 other Tesla
Missoula...