You say "It's about time!" when something finally happens that you think should have happened a long time ago. When you use this, you sound like you're annoyed by how long something took. Here's an example:

A: Pete finally sent me the files I've been waiting for.

B: It's about time!

When you pronounce this, put stress on both "about" and "time":

It's about time!

People don't usually use this phrase in written English.