If you have a fuel-injected engine, it's important to change your fuel filter at least once a year. But if you have a carbureted engine, you can get away with changing it every other year - or even once every two years.
A fuel water separator is designed to remove water and particulates from your fuel before they can reach your engine. Water can enter your fuel tank in a number of ways - through condensation, rainwater or even from using contaminated fuels. Particulates can come from dirt, rust or debris that's been stirred up in the tank.
While most modern engines are designed to run on relatively clean fuels, there's still a chance that water and particulates can make their way into the system. That's why it's important to change your fuel water separator regularly. Depending on how often you use your engine and what kind of conditions you typically operate in, you may need to change it more or less often.
As a general rule of thumb, it's best to change your fuel water separator every 50 hours of operation - or at least once a year, whichever comes first. If you frequently use your boat in saltwater or muddy conditions, you may need to change it more often. Likewise, if you don't use your boat very often, you may be able to stretch out the intervals between changes.
At the end of the day, there's no hard and fast rule for how often to change your fuel water separator. Just be sure to inspect it regularly and change it when necessary - your engine will thank you for it!