Where do I get a US 1 million dollar bill authenticated and how long does it take?

(A) You've come to the right place and

(B) Authentication takes about 2 seconds

The reason is that there has never been a genuine US $1,000,000 banknote. ALL so-called "one million dollar bills" are simply novelty items that sell for a few dollars in gift shops and similar online sites.

The largest banknotes ever put in circulation were worth $10,000. The last printing was in 1945, and banks are no longer permitted to distribute them. The largest US bills ever printed were worth $100,000. These were only used by the Federal Reserve System for internal transactions and it's illegal for private citizens to own one even if you can afford it.