Everyone is different. I start feeling altitude at about 4,000 feet. It took me 2 days to get acclimated to Boulder (5,000 feet), but during our trip we also went up 8,000 a few times and to Rocky Mountain National Park, at 11,000 feet. I was tingly, sweaty, and kind of pale in the national park; I had to insist we leave, and we weren't even hiking!
I did do a short ride in Boulder, after we had been there for a few days. It was OK. But, generally, I don't plan any vacations at altitude, because I just feel awful.
Drink lots of water and no alcohol.