Meaning:

You say "It's about time" when you think

     1. something should happen or should have happened. (It hasn't happened yet.) or
 
     2. something should have happened earlier than it actually happened.

You often use this expression when you are annoyed.

Example:

Do you think Jane has gotten home now?
It's about time.

Mary and Scott are getting married next week.
It's about time! They've been dating for years!

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