I have the Aqua Clear filter. I never replace the sponge and I never replace the bio-cubes (unless they become so worn out, at some point in the future). As for the
activated carbon, I don't use it at all as I perform frequent and large water changes. Just rinse the sponge and the bio-cubes in tank water during your filter maintenance. That is all
The reason the manufacturer tells you to change every medium every 3 months (or every 1 month in the case of activated carbon) is because they want to keep making money off you ... If you do not buy something every 1 - 3 months, they make no money. It's that simple. It's good you've asked that question
P.S. Although, with respect to the activated carbon, you shouldn't keep it in your filter for a long time because it works only for the first 1 - 2 weeks, then it becomes useless. And while you're still keeping it in your filter in that state, I have heard it may start leaching the toxins back to the tank (don't know if it's 100% true but it makes sense to me). If you don't want to buy new activated carbon every month, you don't have to have it in your filter at all. But make sure you perform regular water changes and that your tank is not overstocked.