how long will Camino Frances take

Hey there, I think 40 days will be enough provided that you are reasonably fit and also have factored in some days rest and also that you may get injured and be forced to rest.
It is a good ide to consider that you may get an injury or become ill. Dont mean to be negative but if you plan for this in advance it will be less of a trauma if it does happen.
I did Burgos to Santiago in 14 days and it was a push. But I enjoyed every second.
I wore a pair of shoes, Salomons, not boots. Theye were excellent although I should say that from burgos there are few difficult climbs apart from Cebreiro. Form SJPDP there is the napoleon reoute to ronscevelle which is a climb and would require some solid footwear. I am about to go from SJPDP to Burgos and I will be wearing again shoes,North Face type and I confident they will be OK.
Also last year I had tendoitis and I loosened my left shoe to aleviate the pain. My point is that I was wearing shoes not noots and they were very loose on me.
In any case make sure to wear the footwear, Boots or shoes, make sure to break them in before you go.

I am starting in september and finishing in september so I cannot say about the weather in october!

Hope this helps

Peter