I didn't quite make it to the top of Rainbow earlier this year, so I'm going to give it another shot. From McHugh it's a 3-4 mile run to where the trail to Rainbow branches off. From there it's mostly hiking to the top. Elevation at the peak is 3543'. I expect the whole thing will take 4-5 hours. Here's one of many websites with info on Rainbow... http://www.summitpost.org/rainbow-peak/618183
If the weather is poor, I might reschedule for another day.