How long to get cat dander out of a house?

Question:My allergies are killing me this year. I just moved into my fiancee's house and allergies are awful here. There were 2 cats living here up until about 3 months ago. Most of the floors are hardwood, but there is some carpet... Could it be coming from the ducts? We've cleaned everything and changed the furnace/AC filter...

Answers:
I'd suggest having the duct work professionally cleaned. Make sure the vacuum you are using has a hepa filter on it and change it often. Make sure to vacuum the furniture as well as the carpet.