A great question, nothing specific in my owners manual about a change, and as you say a system flush would be a bear. System on my 99 says ATF, so color could be best indicator. I imagine that because of the limited use and exposure to heat the fluid would not really break down, but contaminants could be an issue , because the tank is vented.
Guess you just put an item on my ,to be checked list.
I'm off to the HWH, web page.
Web page under FAQ, says only if contaminated.
If there is any water in the system it will be milky after use, so if you level then retract, check the fluid for anything but clear. From what I read online newer systems use a hydraulic fluid .