"Ah, about time!" is usually said when someone has been waiting a long time for someone else, or some thing.

I was waiting to pick up a friend at 1:00 yesterday. She showed up at 1:20. When I saw her I said, "Ah, about time." Meaning it was about time she showed up. She was 20 minutes late! :)

When someone, or something, is late we say (facetiously): "Ah, about time!"

When it's time for a glass of wine

The speaker has been waiting for something to happen, and it finally is NOW happening.