When are Open Orders Adjusted by the Amount of the Dividend

Straight from FINRA (emphasis added):

A member holding an open order from a customer or another broker-dealer shall, prior to executing or permitting the order to be executed, reduce, increase, or adjust the price and/or number of shares of such order by an amount equal to the dividend, payment, or distribution on the day that the security is quoted ex-dividend, ex-rights, ex-distribution, or ex-interest, except where a cash dividend or distribution is less than one cent

So the order will be adjusted on the ex-div date.

And when will my order revert back to BUY stock XYZ for $100?

It won't - why would it?

I understand that open orders are adjusted by the amount of the dividend (FINRA Rule 5330). But when are the open orders adjusted?

If the ex-dividend date is Sep 14 and the dividend is 50 cents. then share price will be reduced by 50 cents overnight before trading resumes in the stock on the 14th. Simultaneously, your option limit order will also be reduced by 50 cents to $99.50

And when will my order revert back to BUY stock XYZ for $100? I am in USA.

Your limit order will not change unless you decide to change it, assuming that there are no other subsequent corporate events such as dividends or a stock split.