If you look on the list of visa types that are entitled to an EAD based on the visa status alone, you will see K1 is on this list. This means a long time ago when applications didn’t take months to process, a k1 could come, apply for EAD and work up until the 90th day. They would need to submit another EAD before the 90 days when they do AOS because they K1 EAD cannot be extended or renewed because the K1 status is expired. So you would need to file another EAD application based on AOS status, this is why if you file EAD with AOS, the fee is free, because by the time you get it, you usually get your Green card a few weeks to a month later, which negates the purpose of the EAD because you don’t need it anymore.
So long story short:
-you can get EAD based off K1 status alone
-it will expire the same day your I-94 K1 status expires
-it cannot be renewed or extended
-the only way to get it before the 90 day k1 status is up, is to expedite it, and then expedite the expedite.
-It can go a lot quicker if your fiancé already has biometrics on file from a previous visa or petition, so they don’t need to schedule a biometrics appointment.
Our 90 days was up this past weekend, we expedited the second EAD, based off having a current job, when we filed AOS paperwork and it should arrive in the next couple of days.