I don't know that they ever do. They're supposed to be a constant reminder of how badass you are, or something like that.
On the other hand, it might be one month, and you cant revisit the cell they're in during that time period. Try traveling to a different city from falkreath and waiting for a good month-month and a half, and see if that makes them disappear. If that doesn't, nothing that's been programmed in will.
If you really, REALLY want to get rid of the, and are on the PC, then you can use the console. Press ~, select the dragon corpse, and type "Disable". This will make them disappear instantly and permanently.