You're telling me is a strong way of agreeing from experience. You're telling me, I was there for six hours.
It agrees while claiming to know more about it than the speaker does. No answer is wanted, since it is emphasis rather than a question.
1phraseyou're telling meA strong way of agreeing from experience.
“When he complained about the parking she said you're telling me.”
you're telling me is a phrase.
Practised in Small Talk & Everyday Phrases