what does "hitting the hour mark" mean in this conversation?
a: How late is he?
b: Hitting the hour mark. Hope there's nothing wrong.
The expression, to "hit the ______ mark", where there's some time unit in the blank means that's the amount of time that has passed.
In this example, present continuous is implied with "-ing" and the full sentence is: "It's hitting the hour mark." This means almost one hour has passed since he became late, or, more naturally, he's nearly an hour late.