How much time for that level of adaptation, and how long do you think it lasts? I spend a week staying at around 8,000 feet a little over a month ago and did a lot of climbing in the area and by the end of the week my sustained power at that altitude was much closer to my sea level power, but I feel like I lost the adaptation just as quickly.
Depends on the person, but a week would be pushing it for most people, two weeks is better, three is probably ideal for adaptation. Losing it is probably in the same time frame; for me it's around two to get decent and I can hold onto it for 3-4 weeks.