Just give them a quick squeeze in a bucket of tank water when they start to clog. How often that is, depends on how big the tank is, what fish are in it, how much you feed, etc., etc.. All tanks are different.
I can get away without cleaning mine for more than 2-3 months on a good run in my small 50 litre tank. This is very helpful for you benificial bacteria coloni and in turn fish.
If you dont over feed and remove the uneatten foods on a good filter you wont need to change very often just keep an eye on the flow rate and as said give it a light squeeze in tank water to unclog if the flow slows dramatically.
It's advised you change active carbon this often too but you need a filter with more than one sponge or two filter to alternate changes on or you will be spiking your tank with every new sponge you put in.
Read up on active carbon, but it is ok to use the sponge for longer it just looses the added benifits such as clorine and med nutrifying.