@sjonnie - Change the fluid when it starts to look bad, that means way before you have “burnt material” in it, usually the fluid starts to look brownish first.
Unfortunately many cars these days have sealed transmissions where there is no dip-stick to check the oil.
My Toyota and wifes Lexus both have this type of tranny and both use the WS fluid and it’s considered a life-time fluid. And with this life time fluid my 4runner has a little over 200k miles and I’ve changed the Life Time fluid 4 times. I do NOT like keeping the fluid in there for that long.