How long does it take to be transferred from the National Guard pay system to the Active Duty pay system?

You are accessed into DFAS as an AD Soldier when you inprocess finance at your gaining installation, not at the MEPS. So that means the clock doesn't start counting until you turned in all necessary documents to finance as part of inprocessing. DFAS sends the pay data eight days before payday. If your information wasn't up to date prior to that date you don't receive that paycheck.

So, finance has to receive your documents, send your accession, DFAS has to access you then transmit your pay prior to eight days before the pay period. If all that is completed by today you'll be paid on the 15th. If not, then the 1st of January.

Go down to AER and get a loan, you'll receive all your back pay in one lump sum

Talk to your Chain of Command about arranging you a trip down to Finance. While it's a financial challenge for you, depending on how your enlistment hit the pay cycle, there may not really be an issue. However, I would still check with finance to make sure.

Work with your leadership and S-1. They will help direct you to the folks you need to speak to and work through to fix this.