If we want to emphasisSpecial importance or attention given to something. E.g.: All I wanna do is have some fun.e the length of the period of timeThe length of time between two points of time. that has passed since a past eventA general term expressing that somebody or something does something or that something happens. E.g.: An apple fell on Isaac’s head., an introductory it is used with the present perfectThe aspect expressing completion. E.g.: He’s finished university. tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses..

It has been years since I last met him.
How long has it been since you met him?

In everyday speech, however, it is more common to use the present simple instead of the present perfect tense.

It's years since I last met him.
How long is it since you met him?

Present perfect with FOR and SINCE
Present perfect Present perfect continuous