"It's about damn time" is a phase I first heard used by my Grandpa. It is a wonderful phrase, and it has many, many applications.
The first application is as a statement of gratitude because something has finally happened that you have been waiting for. Example-(this is the way I heard it) "It's about damn time you got out here! We're gonna be late for church!)
The second application is as a longing for something to happen that you have been waiting on for a while. Example- "It's about damn time they bring back Crystal Pepsi."

I have heard many variants on this statement, such as inserting random expletives in place of damn.