Almost 0. If you are talking about the Last Modified Date which many people display on their page, and it updates as you make changes to the page, it has no weight for SEO. Neither it helps in keeping the pages fresh as it is a very small amount of text.
What I meant is the last modified header, which isn't visible. header( 'Last-Modified: ' . gmdate( 'D, d M Y H:i:s' ) . ' GMT' ); PHP: