5.5 hrs with bathroom stops and gas stops
Well, I've done it in 8 hours and that's with stops for 2 meals, a cig break every couple of hours, and perhaps 4 or 5 bathroom breaks (I'm old). I have friends who've accomplished it in considerably less time. I'd say the average is between 5 - 8 hours. Also depend on your route to some extent, and the weather in the passes if you use them. My daughter, Auntimaime, might know this better than I because she lived in Spokane and visited us in Vancouver, WA (across the river from Portland, OR) several more times than I drove in that direction.
Go to mapquest.com, then select the option for driving directions, type in from ( the address that your leaving from or just the city) to the city or address that your going to. After that it will give you directions and estimated time to get there.