I personaly take on most " out of the ordinary" accounts.
BUT, they pay for it, e.g.
a twice a year clean would be charged 2 to 3 times normal rate.
I would do the clean when it suited me & not them.
If the time-scale doesn't suit me then they pay, not me, simple.
If they don't agree then i've lost nowt, if they do then it's extra bucks for me & now'n again, when they see what a difference clean windows make they ask for a more frequent clean (only now'n again mind).
Turn a no-go job around to your benefit, nothing to lose IMO.
tony