(My,) how time flies.
1. Time has gone by quickly, it is time for me to go. Bill: Look at the clock! Mary: How time flies! I guess you'll be going. Bill: Yes, I have to get up early. John: My watch says it's nearly midnight. How time flies! Jane: Yes, it's late. We really must go.
2. Time passes quickly. (Said especially when talking about how children grow and develop.) "Look at how big Billy is getting," said Uncle Michael. "My, how time flies." Tom: It seems it was just yesterday that I graduated from high school. Now I'm a grandfather. Mary: My, how time flies.