My understanding, perhaps erroneous, is that it can take up to a month at altitude for your body to start producing more red blood cells, which is why Everest climbers stop at base camp for a while before going up. However, I did think that there was also an
intermediate adaptation that can occur within a few days. When going to Cusco, Peru, they suggest waiting 2 days in Cusco (elevation: 10k) before going on any of the big regional hikes. They also strongly suggest that you take acetazolamide, which as I understand it is basically the one medication that does anything: it reduces the pH of your blood back down to normal levels, but it takes ~2 days to take effect. Maybe that's why a few days at altitude can be an initial help; it doesn't boost your red blood cell count, but it can be enough time to negate respiratory alkalosis (Thanks,
wikipedia!)