I drank an alcoholic beverage about 12 hrs ago. How long should I wait to start taking Flagyl?

Although it doesn't specify when you should take metronidazole after drinking alcohol, I would recommend perhaps waiting for another 2 and a half days before taking metornidazole.

Do not drink alcohol while you are taking metroNIDAZOLE and for at least 3 days after you stop taking it. You may have unpleasant side effects such as fast heartbeats, warmth or redness under your skin, tingly feeling, nausea, and vomiting. Check your food and medicine labels to be sure these products do not contain alcohol.

https://www.drugs.com/food-interactions/metronidazole.html