the number i have heard is that there is about 1/4 troy oz of silver in a gallon of spent fixer ...
so depending on how often you use a gallon of fixer to exhaustion ...
you don't need to wait for the cathode to be full to send it in ...
if you want to send it in with less, its up to you.
or ... if you want to just hang on to the cathode when it is full
get a new one and stock up on silver until the price goes even higher,
or sell it to a jeweler, or a chemist or put it in a safe deposit box, that is fine too
the main thing is that you are getting it out of the waste stream.