Maybe in a decade or two......
Fluid in the reservoir doesn't mix readily out to the cylinders in any reasonable length of time, at least not in any vehicle that I have owned. Moisture may dissipate through the fluid to some extant, but I can't see it being material. Back when ATE had fluid available in blue and gold, I would occasionally top off with one or do a master cylinder flush (on German cars with shared master reservoirs) and when I got around to flushing the brakes, the original fluid was still in the outer lines and cylinders.
Also, cars may have residual pressure valves, "g" or other proportioning valves between the master and wheel cylinders that restrict or close the system at rest.
You will get more fluid moving between the master and wheel cyls on drum brakes, but, short of a leak, there should not be much fluid moving between the reservoir and the master aside from the little squirt you see on some cars when the master starts to move before the tip valve closes. Very little fluid seems to move on disc brakes.
