I recall some advice that I should change ATF in our former Hurth 50, every year. So I did that, but when we were not using the boat too often would wait a couple of years. Our mechanic advised that we should watch for
any color change in the semi-clear fluid, and we never observed any. We would average about 100 hours a year, run time.
Having never had a measurable leak I have not had to "top it up."
That said, our former Hurth did have a minor seepage around the bottom drain plug . I added a "bonded washer" and it stayed dry from then on. There was supposed to be an aluminum washer there, per the drawing, but it was missing when we bought the boat.
As for the exact fluid level, I always figured that if it was at the mark or a tad above, it was fine. I would not hesitate to add a little bit to keep that level at the indicated point.
I always carry a quart ATF container on the boat anyway, along with a spare gallon of lube oil.
On our new transmission I have been changing it every year just to be sure staying on track for any possible warranty hassle.
So, when in any doubt, just change it yearly. There is a thin tube on our vacuum oil changer that can reach to the bottom and remove most of the ATF. I realize that while, with difficulty, I could drain the new transmission into a shallow pan on our boat model, this may not be possible on your model. I have been using the vacuum oil changer for our trans, since it was installed in 2018, just because the drain is even a more difficult to access than the former Hurth. (!)
Washer Link:
https://ericsonyachts.org/ie/ubs/hurth-17-mm-drain-plug-washer.213/