How long do the aura's remain?
That depends on the level of evil. Use the standard levels of evil for detect evil.
Lingering Aura: An evil aura lingers after its original source dissipates (in the case of a spell) or is destroyed (in the case of a creature or magic item). If detect evil is cast and directed at such a location, the spell indicates an aura strength of dim (even weaker than a faint aura). How long the aura lingers at this dim level depends on its original power:
Original Strength Duration of Lingering Aura
Faint 1d6 rounds
Moderate 1d6 minutes
Strong 1d6x10 minutes
Overwhelming 1d6 days