Have not changed mine in over a year as my membrane is over 8 years old and not worth protecting anymore.
On a new membrane I would change them every 3 months and then take it down to every 6 months, when the membrane
gets more than a few years old then I don't pay much attention to when I change them.
Remember a new 4040 membrane costs around £250 quid and is supposed to last 5 years, you don't want to be spending
£500 over those 5 years just to protect it which you could easily do If you buy the fibredyne pre filter and change it at
the recommended limits.