If your filter doesn't contain any other biomedia then yes your bacteria will mostly be living on the sponges. These don't need to be changed every 28 days, in fact many people advise never changing them at all. You can give the sponge a gentle squeeze in a bucket of tank water when you do your tank maintenance and this will keep it operating efficiently without losing too many bacteria.
Filter wool (floss) is fine mechanical filtration and does need to be changed more frequently, although again you can squeeze it out in old tank water during cleaning to prolong its life.
Carbon sponges (black) will last about six weeks before they need to be replaced.
Your green sponge, I'm not sure if this is a nitrate sponge as they are often green? I'm not sure how long those last for but they are usually unnecessary if you keep on top of regular tank maintenance.
