it depends on the filter and the load its taking really, if it wasn't for having so many juvi fish growing on in my big tank, i probably wouldn't have to touch the filters for 6 months or so, just go by the flow, if its getting restricted, give it a clean
should be fine for a few months, just keep an eye on the water flow as scatz says, then clean when flow is slower (but dont clean too well, leave some bacteria in it!)
I very rarly strip and clean a eheim filter as they are working at a Bio level, I would expect very little rubish in them .
As stated the load on the tank is the key.
If you were to use a sponge, use a coarse one. I had a fairly fine sponge on my eheim 2329 (I broke the strainer needed a standby)and after a few days it got too clogged and starved the filter.
Another possibility is the coarse dimpled foam sheets, just cut to size and roll into 2 layers (dimples out), these are very flexible. I used to just tie them on with cable ties, but the last couple I've sewed up with some nylon thread, a neater job and it only takes a couple of minutes.
<http://www.ketteringkoi.com/acatalog/Filter_Foam_Sheets.html> probably cheaper on Ebay.