Peter Westwood

This plugin allows your users to configure a reminder to be sent to them if they haven’t posted in the last n days.
You configure the reminder on your Profile page (or the profile of another user if you have than capability)
It is in the Personal Options section and called Blogminder Threshold

PJW Blogminder

This plugin allows your users to configure a reminder to be sent to them if they haven't posted in the last n days.

brendannee

Keep your site’s content fresh. This plugin adds a box to the WordPress admin dashboard that lists all pages that haven’t been updated in recently. Every time you log in you’ll be able to quickly see which pages might need review to ensure the content is still correct and everything is up to date.
You can set the threshold in days for considering a page “old”, the default is 90 days.

Old Page Reminder

This plugin adds a box to the Wordpress admin dashboard that lists all pages that haven't been updated in 90 days.

PJW Blogminder vs Old Page Reminder