MrPillow is right. You should replace your filter media only when it is basically destroyed and useless. The bio-wheel should never have to be replaced. Filter media should only be cleaned when your filter is noticeably clogged or discharging dirty water. If it's not dirty, don't clean it. You might go a couple months between filter cleanings depending on your situation. You want to tamper with your filter media as little as possible so the beneficial bacteria doesn't have to re-grow after some of it was washed away.
Never clean all your filter media at the same time. For example, if you're using a sponge and nylon floss as your media and you notice that it needs cleaning, clean the sponge one week (while leaving the nylon floss alone), and clean the nylon floss two weeks later. That will help make sure you don't remove too much beneficial bacteria at one time.
Another thing to consider is the various types of media you could use. I have the same brand of filter that you do, so I know these filters come with "cartridges" that consist of filter pad and activated carbon. The reason your instructions are telling you to replace the cartridge every month is because of the activated carbon (and because they want a piece of your wallet). However, activated carbon isn't necessary unless you want to remove medication from your tank. I would suggest scrapping the filter cartridge all together and finding your own filter media. Use at least two different kinds, preferably one providing mainly "physical filtration" (removing visible debris from the water), and the other providing "biological filtration" (an ideal habitat for beneficial bacteria to grow).
In my bio-wheel 100 filter I'm using a bio sponge (provides both physical and biological filtration) and porous ceramic filter rings (mainly biological). My sponge will eventually fall apart and need replaced, but the ceramic rings only need rinsed every once in a while and might never need replaced. There are lots of different filter media options, so look for what will work best for you.