Do you have a nitrate test? A good way to tell how frequently you need to change water is to watch the nitrate level. Take it down to 10 ppm and when it gets to 40 ppm change water again. A month or so of watching should give you an idea how frequently you need to do it.
The more different you tank's water parameters are from your tap water's, the more frequently you need to change water just to keep them close. Don't do 75% water change when the pH is far apart. The more often the water changes, the smaller they can be.
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but I get told to change it every month?
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Changing the cartridge every month is necessary to keep the carbon working. If you don't feel you need the benefit of fresh carbon (no cloudy water, no oily film on water), you can just rinse cartridge in water change water and put it back. Replacing the filter cartridge does cost you a portion of your bio-filter, so you should have a reason to do so.