The instructions on the cartridge to change the filter pad every month is just the company's way of making more money. All the filter needs is a gentle rinse in old tank water when it starts looking really clogged--OFL is right, there's no need to replace it unless it's actually falling apart.
I use carbon in my filters, many cartridges come with some carbon, but usually it becomes ineffective after about a month. Hence the instruction to change it. However, I just cut a little slit in the cartridge and pour out the old carbon, then I replace it with new higher quality carbon. Seachem Matrix Carbon lasts a lot longer than the generic stuff, so if you're planning on using some carbon, I recommend that brand.