as a carpet cleaner also we tend change them more than wfp - all we do is replace the pump heads , you notice a drop in pressure out the end as a tell tale sign , this comes about after running too hot water through them over time, not really a problem with wfp.
If it helps i have a flojet 100psi for wfp that is over 5 years old and gets used 5 days a week - still going strong so they seem to last.