Once approved, how long is the I-129F Petition valid for? 
Approved I-129F Petitions are valid for four months.

Once the K1 Visa Fiancee Visa is issued, how long does the fiancee have to enter the United States?
A K1 Fiancee Visa is valid for a period of six months and it is used for one entry into the United States.

Will the fiancee be able to work once they enter the United States on a K1 Visa?
Yes. Once they enter the United States on a K1 Visa, they may immediately apply for a Work Authorization Permit. However, since the fiancee must re-apply for a Work Permit after the filing of an adjustment of status (which is required after the marriage), we generally recommend waiting until after the marriage takes place.

Can the Fiancee's children come to the United States prior to the marriage?
Yes. Children are eligible for K2 Visas which would allow them to accompany the Fiancee or follow the Fiancee within 6 months of visa issuance. If the children do not apply for a K2 Visa, they may also be eligible for a K2 Visa after the Fiancee is married as long as the children apply within 1 year of the Fiancee's K1 Visa issuance.

Can U.S. Citizens apply for more than one K1 Visa Petition?

A U.S. Citizen may not petition for more than one K1 Visa at a time. However, if the marriage from a prior K1 Visa does not take place and the fiancee leaves the U.S., the U.S. Citizen may petition for the same fiancee or another one in the future.
Is the K1 Visa issued immediately following the embassy interview?

Each embassy has its own unique procedures. Some issue the K1 Visa the same day and others process it within a few weeks and have it mailed by courier.

Can a Permanent Resident use the K1 Visa Procedure?
No. You must be a U.S. Citizen to be eligible to file for a K1 Visa Petition.