Comment 11 for bug 1219337

A somewhat sensible workaround I can find at the moment is to force re-authentication every time you type sudo. The way to do this is by adding:

Defaults timestamp_timeout=0

to the Defaults section of your /etc/sudoers

This will work on Ubuntu, OS X, and other variants.

Details can be found in http://www.sudo.ws/sudoers.man.html

We really shouldn't be trusting the clock to being with. The fact that Ubuntu developers have seen fit to add "convenience features" to bypass security rather proves the point.