About Time

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'At last!'

'(It's)about time!' is said with relief or sarcasm when somebody has been waiting a long time for something to happen.

Rover

How would you use it in a sentence?

- The pizza guy is here.
- It’s about time! / About time!

"It's about time you showed up!" - Said to somebody who has arrived late.
"It's about time you got a haircut!" - Said to somebody who, in the speaker's opinion, is overdue for a haircut.

"It's about time the council repaired that road." - A complaint about the council's lack of action.

So in short, we can use 'about time' in place of 'at last' or 'eventually' ?

Like, you came online about time.

?

`
No — that's not the same at all.

You would have some exasperation or sarcasm in your voice.

'So you've come online at last, have you? It's about time!'

Rover

Huzaifa, Just punctuation, You came online? At last! (A question, followed by an answer) or "You came online at last?" The 1st has a little dig and suggests someone hs been promising to do something, the second is subtler and more polite and hints tht someone did something they were unsure of. Both ask "Why did you take so long?" but how you present it is the difference.

What I have understood from your answers is it somewhat means EVENTUALLY or ATLAST but it's more used for sarcasm or irony.
right?

Not always.

It's about time you got a haircut! I am saying that you should have a haircut now; perhaps you should have had a haircut before now. There is no way that 'eventually' or 'at last' are synonyms here.