It's about time...

It's about time!   
=  It's almost too late! 
=  Sb. has been waiting a long time for it!

Think about THESE GUYS' MOTHERS...
I'm sure she thinks:


It's ABOUT TIME this guy here below STOPPED playing with that.

and
It's about time!   
=  It's HIGH TIME!

It's HIGH TME the girl below realized something...
Don't you think? 

It's about time you understood it!     [  It means you still can do it!  ]