What are your plans for this roadtrip ? Is it just to get to Denver as quickly as possible or are you planning on sight seeing and taking detours along the way ? How long do you have ? Going direct will mean an overnight stop along the way, as Buck mentioned, you shouldn't plan on covering much more than 550 miles in a day.

Once you have figured out how far you want to get on the first day and what you might want to do along the way will help determine the best time to hit the road.