Just have to put my 2 cents in. I also use aspen in a 2x4 with 2 piggies and I change it every 2 weeks. We all know how piggies have favorite places to urinate and mine are no different. 3/4 of the bedding is dry and clean and usually 1/4 is wet.
In the beginning I was changing the bedding every week, and I thought what a waste this was. Most of the shavings were still dry and good. They would usually urinate as soon as they when down the ramp and naturally by their hay, the rest of the bedding was brand new.
So now I change it every 2 weeks, and there is still alot of clean bedding that is getting dumped, and no smell whatsoever.
Let me mention though that I have a 3 level cage so there may be some urination done on the upper levels even though its never wet up there.