when should I replace the pierburg fuel pump in my rotax 914 engine

What clogged your Fuel Filter?

The Pump is a Rotary Vane type.
Minor wear of the vanes over hundreds of hours is to be expected.
If the pump has shed enough material to clog a fuel filter I would expect the pump to no longer be working.
A brand new pump is rated at 120lph (31gph) Open Flow at Zero Pressure and 1800hpa (26psi) into a static load (Zero Flow).  

The engine needs about 6gph Max at 10psi Max.
If your pump can still move 20gph ( 1 quart in 45 seconds) into an open container and make 20 psi into a blocked hose, I would call it serviceable.

Thank you.  I don’t know for sure exactly what the composition of the particles are.  They are not gritty to the touch, about the size of very fine sand.  Under the microscope most of the particles are tan, fibrous appearing, and about a third of the particles are black like you would expect to see if a rubber fuel line was being chaffed.  I replaced the rubber fuel lines a year ago.  I flew the aircraft about 50 hrs last year