You can use xautolock, it won't log out but it'll ock the screen...

On Tue, 8 Apr 2014, Michael Moore wrote:

> I'd like to have XFCE automatically log out after a period of inactivity. I
> don't have or want power management on this computer since it's also a home
> server and I don't want it to suspend. It seems like everything I'm finding
> says to tie auto-logout to the power management scripts. 
> Does anyone have any suggestions? 
> 
> Thanks,
> Michael Moore 
> 
>