I always keep the remaining amount on my shelf...until I need to use brake fluid...and then I buy a new bottle because the one on the shelf is two years old.

Like you, I keep the remaining for potential top off's, but the truth is on our motorcycles that have a sealed reservoir, if you need to add fluid you probably need to repair a leak first.

I buy the synDot4 in pints.  takes 1/2, and it's not sitting around so long.

I used some in my Camry recently, that had been in a sealed bottle for some years, didn't seem to hurt things.
This doesn't apply to S40 owners where you can buy new brake components, but don't use silicone fluid in wheel or master cylinders that have been re sleeved. It finds it's way out.

That is correct ,when dark or cloudy it needs changed.

If everything is sealed then the brake fluid will be good.
All fluid has "some" water in it.
I worked at Chrysler Canada as a Pipefitter in the 1960's and spent some time working the "tank farm" - that is where the storage tanks were located by the railway tracks near the plant. There was tanks for brake fluid, motor oil, transmission fluid, anti-freeze etc.
I remember each rail tank car with brake fluid had a sample sent up to the engineering lab to test for the amount of water in the fluid before off-loading it - a certain amount of water was acceptable ( I forget the cut-off value).
Modern master cylinders and calipers are sealed, there is no need to remove the top of the cylinder as we can see through the sightglass to check level.
I have never changed out brake fluid in almost 60 yrs of driving. Most vehicles were kept 5-10 yrs and one was kept 14 yrs.

Ruttly, that's hilarious!