Even a six day program is really a minimum amount of time to get acclimatised enough to climb Mont Blanc. Acute Mountain Sickness (AMS) is common at alpine altitudes and should not be confused with mild altitude symptoms such as headache and breathlessness. AMS is extremely debilitating, comes on suddenly, and leaves you barely able to walk or look after yourself a person suffering from AMS is considered a medical emergency, and if it occurs in a remote position on Mont Blanc and/or in bad weather, it can have serious consequences for all concerned.
In 2005, one of our courses coincided with an attempt by seventy French soldiers to climb the mountain without previous acclimatisation they were all aged between 20 and 35 and very fit. We counted 25 of them suffering from AMS at the Gouter hut (3800m), and a similar number at the Vallot Hut (4360m) all of these failed to reach the summit and some needed assistance to descend, while we counted only 16 of the original 70 as having made the top. None of our 8 clients developed altitude sickness that day. This was an interesting experience as it gave a rough idea of the likelihood of developing AMS when attempting a two-day ascent. On our six day courses, we have found about one in twenty of our clients has had to turn back because of altitude problems, and these were usually people already close to their physical limits and therefore pushing their cardiovascular systems very hard.
One can still find three and even two day ascents on offer which can seem very attractive as so much less time and money is involved. However unless you know you are properly acclimatised (at least 2 nights sleeping high including several hours arduous exercise above 3000m beforehand) this will very often prove to be a false economy - just ask the French army.
Whilst exercising high is obviously important, sleeping high is also a vital part of acclimatisation, as during sleep respiration depth drops, therefore hypoxia and adaption to altitude increases. Aside from pre-acclimatisation and certain drugs, the best defence against getting sick on Mont Blanc is simply to get yourself as fit as you possibly can.
Treating AMS with a Pressurised Gamow Bag