what kind of filter do you have?
I'm going to venture you mean the hanging type, and I'll respond with that in mind. I have a couple tanks with hanging filters and each week when I do a water change, I turn off the filter, remove the cartridge and swoosh it around in the bucket of old tank water. This is to 'get the big stuff off' so that water isn't being backed up in the filter. Anytime your filter overflows where it's not supposed to, this is what I would do. If you find yourself doing this several times a week, you may need a bigger/better filter.
The reason to rinse, not replace, is at least two fold. Of course, this means your not buying a thousand cartridges a year, and also the beneficial bacteria that your tank needs lives in the filter media (as well as substrate, etc). Losing this source all at once isn't the best idea.
Then naturally at some point it will get worn out. At the time it is visually worn out replace it. If your concerned with a mini cycle happening (due to the beneficial bacteria you removed) reduce feeding and unless you have ultra sensitive fish, you should be fine.
There is a thread in the chit chat forum discussing carbon (which is in the filter cartridge). Personally I change it every 6 months. Three are others who believe after two weeks it becomes ineffective and some whom say it leaches contaminants back into the water at some point . It seems many even opt to not use carbon all together. Choice is yours, but honestly, none of the mentioned choices are 'bad' although some may be better than others.
I think the person who told you that either doesn't know what he is talking about or just wants to sell more of them.
