Hi. In the last few years we’ve done a 3 week trip, then a 6 weeks trip and this year will be about 10 weeks. A few suggestions (the book in the link above is mainly Bike Park stuff that you can find easily enough).
Andorra – great xc, uplifted techie stuff and bike parks – plus it’s cheap
Basque MTB (if you are heading that way = I’ve just booked with them after hearing too many good reviews, and wanting to ride near the coast)
Tende – look it up on Google. French / Italian border in the Maritime
Alps, there’s good resources on the net, and some very technical riding there (not to mention 1000m climbs….)
Riviera Bikes – not the most scenic, but some of the best trails around and you get to do a lot of descending in a few days if you are passing through.
Valloire – nice little town with easy lift access, some handy via ferrata there too if you like that sort of thing.
Saux D’Oulx – a bit weird, but interesting to see how the Italians do things!
Lake Garda – amazing, but could be a bit out of your way?
Then there’s all the stuff in France that doesnt need any introduction – but my list there would include riding the Megavalanche course, La Varda as Les Arcs (Doubleheader is good too), the drive over the Galibier and other big passes, the black /red run down to Venosc, and so many trails around Chamonix.
You’ll get loads of advice on here, but drop me a line if you want any specific info.
You’ll have a great time.