I replace my membranes when I see the TDS tests showing higher than "normal" levels or the water tastes a bit salty. I have found it important to actually test with a meter vs. relying on the built-in sensor. That sensor always seems to show a green light after a few minutes running and I am not sure how it is calibrated.
I have never seen the numbers exceed approved standards (I believe 500), but I like to err on the side of caution if I see TDS numbers rising say above 300. Hence, I replace about every 2 to 3 years, particularly if I do not use the watermaker routinely (winter storage for example).
In 5+ years of being an Amel owner, this will be my second replacement.
Phantom, A54 #44