Are you on 10.1? If so, Conditional Subscriptions can do this for you.
If you're not on 10.1 yet, then consider VizAlerts, an open-source tool developed by myself and Jonathan Drummey, which can easily accomplish this. It is a generic Tableau Server data-driven alerting tool that uses your viz information and piggybacks off of Tableau Server's Subscriptions feature, so once installed, you or your users could build any alert that you liked, so long as you have the data. The caveats are that this tool is NOT supported by Tableau, comes with no warranty of any kind, and doesn't work with Tableau Online. For your needs, I'd direct you to this post on our support forum, which, although designed for alerts when extracts fail, could easily be tweaked for when they run too long.