Acclimatization is so complex that even the scientists can explain it completely. Anecdotal experiences like yours are why and how we make the recommendations we do about these trips. I coach Adrian Ballinger who was on the north side of Everest in April and May, returned home for a month and then summited K2 with out supplemental O2 in July. He felt very acclimated and strong on K2.
David has typically used about 12-14 days on his high altitude training camps in the Khumbu. It is enough time to get a good stimulus without getting too worn down. I think this is highly individual and depends a lot on what you are doing while at altitude.
So, trust in your experience rather than what the nay sayers have said. You know what works for you.