At least 3 hours. You should just be able to hit I-70, to I-76, to I-25 up to Fort Collins. This should be about a three hour trip, but you might catch weekend traffic, in which case you'll spend too much time stuck on I-70 with everyone ELSE trying to get back from the mountains. Once you hit Denver, it is about hour up the road to the Fort.
If you have the time and want a really nice scenic route, then up through Steamboat and down through Poudre Canyon is really nice, but that'd be even longer.