It's all about genetics. Years ago I was selected to be a guinea pig for a study on altitude acclimation conducted by the USOC. We lived at 12K' and trained every day in a Colorado valley at 10K'. We all ate the same food and breathed the same air. Every few days we drove to a lab for blood tests. Our red blood cell counts were especially important. The higher the better. The results were all over the board. After one month some had improved way more than others. I never raced well at at altitude (> 5K') because my body just didn't adapt as well as others and there wasn't a darn thing I could do about it.

Pesonally, I feel acclimated within 5 days but YMMV. And I do think light exercise during the adjusting period is important. Just remember to start slowly. Eight days should give you at least a very good start.

And as a side note, a follow up study was done after our little experiment and all of the high altitude changes to our blood were gone within 5 days of returning to low altitude.

On another side note, Summer+altitude=usually dry air. You won't realize how much you're sweating because it will evaporate very quickly. Drink up.