I run a Fluval 406 on my 55gal with a prefilter (Eihiem) and have been monitoring things for the past few months rather closley as I am growing out juvis.
I squeeze out all sponges, rinse the purigen and bio media in tank water, and thats about it - oh and I clean the impeller and magnet.
The first few months I cleaned them once a month, and they looked very clean inside, the sponges even looked new still, cleaned and replaced polishing floss anyways - I did not notice readings change from before to after.
This last time I let it go for 2 months, and it was much more dirty, still not nasty (like my FX6 on my Mbuna tank with no prefilter...lol) but was dirty enough I am glad I cleaned it. I noticed about 8-10ppm Nitrate rise every day before cleaning, and 3-4 per day over the last few days (just cleaned it this weekend) - so.... I would say every 1-4 months - depending on your stocking and feeding :P
Like Filip said above my prefilter sponge some days is worse than the sponges in the canister after 2 months and I clean that prefilter daily!
And get a prefilter!!