How am I notified a buyer if my option was exercised by the buyer on TWS? How much delay is there from when exercised to when I'm notified?

Traders submit exercise notices during (and after) market hours and their brokers notify the OOC. In the evening, the OCC uses an electronic "Wheel" to randomly determine assignments which are then sent to the brokers who then use their own in house method to determine who gets assigned.

With IBKR, your end of day E-mail statement will contain the details of the assignment transaction. So that means very late the same day to early the next morning, depending on whether it's a normal trading day or expiration. This information would be available in your online statement as well.