That is your prefilter - it is just a sediment filter. MEP's recommendation of once a quarter is a good starting point. Some people will get more, and some less life out of the filter - depending upon how much UN-dissolved contaminants you have in your water.
Most membrane manufacturers spec a 5 micron (or less) sediment filter. The most common sediment filter sizes are:
- 10" x 2.5"
- 20" x 2.5"
- 10" x 4.5"
- 20" x 4.5"
these dimensions are nominal. There are also many other sizes available, but those 4 are the most common.
Russ