When you look at distances, a good rule of thumb (and also easy math to do in your head) is an average of 50 mph. Not that you'll be driving that slowly, but once you figure in stops for fuel, food, bio, etc, it comes out to about 50 mph.
X2. This rule of thumb works very well.
If you don't want to feel rushed and depending on where you go, 3-4 weeks is the minimum I'd plan. I've been out to SD, CO, WY & MT twice and it's a long drive to get out there. You will lose 3-5 days each way just driving, depending on where you go.