When I visited Peru recently, I observed a few tourists who had flow directly to Cusco (altitude 11,152') with an ashen look on their face sucking on O2 in the hotel lobby. The guides say the adaption period is about 2-3 days, less if you spend some time at an intermediate altitude (6,000').
The PPA (limited O2) does not challenge the leg muscles as much as a sea level marathon, so most runners feel pretty good the next day, which is maybe why so many choose to do the double. I live at 8,000' and I notice when I spend some time near sea level that my adaption to altitude is fleeting. Many runners will focus their training in the final two weeks at high altitude. The idea is to challenge the body to adapt to altitude (more O2 efficiently) while maintaining fresh legs for the race. Keep in mind that a full 3-2-1 is 12 miles at an altitude above 12,000' so don't ruin your taper.