It depends on the salinity of the flush water, how often the toilet is used (sitting unused for long periods is much harder on any equipment than constant use)... In the discharge line, it depends on how well the line is rinsed--if at all-after each flush AND rinsed out before the boat will sit...in the intake it depends on whether the system is plumbed to allow sea water to be rinsed out before the boat will sit...i.e. intake line teed into sink drain line...etc.

Best rule of thumb is, clean the air valve about once a month...replace it when it starts to squirt.

And it's more important to maintain the air valve than most people realize. 'cuz a clogged air valve turns a vented loop into an UNvented loop that no longer has any ability to break a siphon. That's why I don't recommend putting a vent line on a loop that squirts...the hose diameter is so small--1/4"--that it very quickly gets clogged with salt, sea water minerals and--in a toilet or tank discharge line--waste, becoming an unvented loop....and because sticking a hose on the nipple solves the problem--squirting--permanently, it's "out of sight, out of mind"...never maintained.