how long ago was the mediterranean cut off from the atlantic ocean

The Mediterranean was cut off from the Atlantic about 5.9 to 5.3 million years ago, during the Messinian Salinity Crisis. In simple terms, that means roughly 6 million years ago.

A bit more context:

• The separation began around 5.96–5.97 million years ago.

• The crisis ended when Atlantic water rushed back in around 5.33 million years ago.

• So the Mediterranean stayed largely isolated for about 600,000 years.

If you want the one-line answer for a post: About 6 million years ago, give or take a few hundred thousand years.