Here are some examples of fuel filter elements in different stages of life. We recommend that fuel filters be changed at least every year or 100hrs. If the filter, when you remove it, looks anything like the two lower photos, you should reduce the time between filter changes.
Here is a fresh, new filter element for a popular primary fuel filter. It has nice, clean pleats on the filter paper. It only has to be immersed in fuel for a short time for it to discolor, but that doesn’t mean it’s in need of replacement.
Here’s a beauty! You can not make out the pleats for all the slime on it. Looking at this filter, we would recommend changing this filter a little more often.
Although this filter doesn’t have the external mess like the one above, it is waisted in the middle. This is caused by the vacuum generated by the lift pump on the engine. This filter clogged and the vacuum was generated as the fuel pump tried to suck fuel through it.