I live south of there, just past the border in Oregon. If you're making a spring trip, mid-May to early June is generally the best time to go, depending on climate conditions that particular year. If you get much earlier than that, then the winter wheat hasn't had much of a chance to show up. Go later and your colors go from greens to yellows/greens, and every shade in between. However, I have seen some great shots that time of the year as well.
It's good that you're planning on spending multiple days. If you get a cloud cover, than you won't get those evening shadows that Steptoe Butte is so famous for. Another place you'll want to visit nearby is Palouse Falls. If clouds are moving on a given evening, you can get some spectacular images from there.
I will likely be making a trip there in the spring, so drop me a line if you make it. I can show you around. Don't pay somebody to show you the place. It's easily accessible if you have a decent roadmap and do your homework first. Glad you're planning on making the trip; it will be worth the effort.