How can you set an auto logout after X minutes of inactivity?

Hi, I’m making a private forum using the latest version of WP 3.0.1. Because it’s a private site, I would like the members to be logged out automatically after a short time of inactivity, i.e. 1 hour say.

It’s important that it’s after inactivity, i.e. no page loaded, rather than a set session length.

What’s the best way to do this, I couldn’t find the answer to this problem.

Thanks for your help