Alkalinity refers more to the hardness level of the water. As for the filters, what do your parameters say? I would go by my water testing to determine if the pads need replacing. Typically you replace carbon pads on a monthly schedule but it really depends on the bioload of your tank. If your water is staying clean longer, then you may be able to extend it.
The sponge filter adds service area for beneficial bacteria to grow. The bacteria that lives in an anaerobic environment... This helps remove nitrates from your water. However if they get dirty with detritus, as they often do, then they can pollute your tank. But if you clean them, then you can remove the beneficial bacteria. This is why I eventually got rid of my filter sponges and hooked up a refugium with mangroves and a deep sand bed.
However if you want to keep them I would go by the water parameters. If you are getting a rapid build up of nitrates, then you need more surface area for anaerobic bacteria. I would add another filter sponge if I could... But if your nitrates are manageable then I would keep the setup you have and just change out the carbon once every month or so and lightly rinse the sponges with sea water as needed to keep them clean.
