How long does pet dander remain in the house?

4 months. About four months for most cat allergen, and dog is probably similar, although there isn't much evidence about dog. Levels can persist much longer, especially if the levels are extremely high originally.
Varies. Cat dander tends to be more buoyant and can actually find itself in the recessed corners of the ceiling. Dog dander is easier to clean overall. Effective vacuuming and throwing away pet beds can help although dander can hang around on clothing for quite some time.