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5-6 days is about the right amount of time to make this trip, assuming no real extra stops, but that does kind of assume you'll see good weather. If you do see snow - a distinct possibility at this time of year - it could make it difficult to meet your timeline, especially if you are not comfortable driving in winter conditions.

Every route you could take on this trip does have a chance of seeing winter weather, but I will say that by going through Denver, you limit your route options. Getting out to Denver, you pretty much have to use I-80 or I-70 (or a combination of the two) and after Denver, I-70 to I-15 is the straightforward route. I-70 across Colorado and Utah is one of the most scenic highways in the world and it is well maintained all winter long, but it does go over a 10,000 foot mountain pass, and has a high likelihood of seeing at least some snow.