Altitude acclimatization is not just a single change in your body but a long list of different things that are going on. There is a nice chart on p. 326 of House and Johnston, Training for the New Alpinism, which gives a list of the following adaptations (they label #6 as two items):
increased ventilation rate
increased heart rate and blood pressure
blood pH increases
decrease in plasma volume
increase in red blood cell mass and hemoglobin concentration
decrease in size of muscle cells, and consequent increase in capillary density
increased density of mitochondria
Your question is about how long it takes to lose the adaptation, but that doesn't seem to have been studied very thoroughly, so I'll start by relating the more well established information on how long it takes to gain the various adaptations and then circle back and try to answer your question somewhat. For each of the items above, House and Johnston give a "time for adaptation to begin" and a "time to end." I think the "time to end" means how long it takes until you're fully adapted. The time to begin is zero for 1-2, a few days for 3-4, hours for 5, and weeks for 6-7. The time to end is weeks to months for everything except 4, which is supposed to be complete in 1-2 days. #3 only happens above 5000 m, so you won't experience it at all from being at those altitudes in the Alps, but you can get the same effect by just taking acetazolamide (diamox).
OK, so what about de-acclimatization? House and Johnston have a short section about this on p. 358, titled "How fast do you de-acclimate?" It seems to be short because there's not a lot of solid evidence. In one study, people spent 16 days at 4000 m, then went to sea level for 7 days, and then went up to 4300 and were tested. "They were found to retain about 50% of their acclimatization." Unfortunately, House and Johnston don't give a reference to who did this study. I would like to know, e.g., by what measure the 50% is computed. In another study, called "Operation Everest III," the subjects lost their elevated hemoglobin levels in only 4 days.
So the evidence is obviously pretty scanty and not completely consistent. House and Johnson say:
When we examine our own experience, look at the historical record of high-altitude climbing, and ask others, we find that the most common rule of thumb is that you lose your adaptation to altitude at about the same rate as you gain it.
Everything I've said so far was general. You have a more specific trip in mind. The big question to ask would be how high you intend to go in the Himalayas and how long you intend to stay there. It seems like the necessary acclimatization time rises very quickly with altitude. For Kilimanjaro, at 5900 m, most people take about 7 days to acclimatize. House and Johnston give some sample acclimatization schedules for higher elevations. For Denali, at 6200 m, they show 18 days as normal, 10 as a "fast ascent." For people climbing to 8000 m, the schedules they show are all about a month. If you're going high in the Himalayas, then realistically you're going to need weeks to adapt, and compared to that, a few days spent in the Alps is probably not going to be significant.
