Hey Stef,
Everyone is different but having been on altitude many, many times, I can say that I don't stay acclimated for much longer than 1 week. If I go to the mountains one weekend, I still do fine the next weekend. If I wait two weeks, I'm pretty much back to square one. One thing that I do find, is that you acclimate quicker if you go on altitude a lot (something you are not doing).
So, doing a big mountain like that in two days and for the first time, seems a tough undertaking. I do remember how you got altitude sick on Mt. Dana here in CA, so I think you'll have a very hard time om Mt. Blanc. That said, if you don't mind turning around when you get sick, it never hurts to try! I've done plenty of high peaks in two days but I also can go from 0 to 4000m without getting sick.
Hope that helps
!