My best friend lives in Spokane so I've driven it multiple times during the winter. It is almost exactly 400 miles. Unfortunately, 70 of those miles are on bad interstate in mountainous Northern Idaho. During the summer you can do it in 6 hours if you take quick pit stops. During the winter I usually drive it in 6 1/2 to 8 hours depending on conditions in Northern Idaho and how long I spend at Rockin' Rudy's.
I don't suppose there are any ski hills along the way, are there? The drive would be much better if I could break it up with a day of skiing on a hill I've never been to before (although it might mean that the Advil consumption goes way up for the last part of the trip).