listening to my breathing on my Muir Trail videos from this year, I think it takes easily 3 weeks to get well acclimated. I never felt sick, never felt slow, never felt like I was being slowed down by lack of acclimation, but I can tell from the amount of heavy breathing in the early videos compared to how little of that is audible 200 miles down the trail that there are significant changes that happen after several weeks at altitude than the 4-5 days I usually give it before even starting the hike.