I am late to this party, but I have experience with this as of today. We wrote and requested to have them transferred 2 days ago, so 48hrs seems like a reasonable data point.
This is what I did:
1. Email the request to cruisenext@ncl.com. I sent it from the profile email address of the transferrer (person sending the cert)
2. Include the names, latitude numbers, and dates of birth for transferor and transferee. I figure the more info they have proving I am who I say I am only helps.
3. Include numbers and dates of expiration for certificates
4. Wait