OK, so just over another 2 weeks have passed and I can say a bit more about the battery life of the MX Master:
As indicated in my first impressions, the first full recharge lasted 7 days.
My second recharge lasted 8 days, right now after another 8 days, the third recharge is on its last legs.
I would suggest the amount of mouse use has a significant impact - less power is used when the mouse is asleep - however 40 days? With my usage not anywhere near it. A heavier user may actually encounter shorter battery life while someone using the mouse less could enjoy longer battery life.