As others have said, the time to get an EAD varies from place to place. Temporary 90 day EADs aren't worth it - just go ahead and apply for the EAD with the AOS. Oh, and one other thing - unless your fiancee plans on working underground, the EAD is a real document, along with the SS card. She will need both to work legitimately in the US, at least initially, until she become a legal permanent resident.