WordPress plugin, configure-login-timeout by n8v makes timeout values user-configurable through the Users admin control panel.
By default, WordPress makes your login session cookie expire in 48 hours (or on browser close), or 14 days if you check the “Remember Me” box.
Requires WordPress Version: 2.8 or higher Compatible up to: 3.1alpha.
Downlaod: http://wordpress.org/extend/plugins/configure-login-timeout/