You will never need to take them out really. Unless you are cleaning the filter. The reason I use the dollar store sponges is to cause water to fall from a higher area than just the normal flow height of the penguin 350 and 200. The more water will hit the wheel, therefore it turns more and more water is on it.
But once again, if your wheel is moving, you are fine.
For carbon reasons probably, but I do not worry about that. Carbon (as far as I know) removes odors and medicine. I replace my cartridges when the blue material starts to get holes in it. Save your money unless you are using the carbon for odors or medicine removal. I have used penguin filters for over a decade and have always only replaced the cartridges when they get holes which is normally 1.5-3 months.
You don't need to change them once a month. Do as Fishbumsuggested,
As far as the cartridges, use the ones you have until they are falling apart,then buy some bonded blue filter pad,and use that instead. Will save you a bunch of money in the long run.
They don't make money off filters, they make it off the cartridges. You can think of it as the "lather, rinse, repeat" of filtration. The more you buy, the more money they make, so of course they're going to suggest you change it as frequently as they think they can make you.
The cartridges are mech filtration before the bio. That's all you need. Bonded blue filter pad or cartridges will both work fine so long as they're catching the particulate matter. Use it up, wear it out, then replace it.