To help give you an idea, I'll give you a comparison between your cage and mine. (You have 28 square feet of hutch space.)
We have six female piggies (two aren't fully grown) in a double level 5'x2' outdoors hutch, which makes approximately 19 square feet (because of the ramp). When we had four piggies in there, I only had to clean them out once a week; I now have to clean them out twice a week. I would say our cage is currently too small for six piggies and we're looking to make them something much bigger. They're not in the hutch all of the time either; they spend all day (approximately 8-hours at the moment; double that in the Summer) in an 8'x4' run. When we went to London for two days, the neighbour who looked after them only put them out on the second day, because it was too wet the first day. I had cleaned them out the day before we went and then cleaned them out again the day after we got back (72-hours after the first clean-out); the hutch was much worse than normal, because they'd been shut in it for a whole day. Therefore, I would say it depends on whether you will be putting your guinea-pigs out in a run or not, as well. If you're not, I'd put fewer piggies in there.
Bear in mind that the bigger your cage, the less frequently you'll need to clean them out.
It also depends on whether you have boars or sows; boars need far more space than sows to prevent them from fighting and you'll need to give them a bowl, water bottle, etc., each (you'll need to get clarity from someone who has boars).
Hope this helps and let me know if you'd like clarity on anything.