Swipe in from the right to get your Charms bar, then Settings, Change PC Settings, Account, Sign-In Options, you should see some fields there relating to password policy. One of them should have a time-out setting.
I first had to "reset the security policies" in order to get this to work as I had set up a connection to my exchange server for email. Now it works fine though the maximum timeout is only 15 minutes. Wish it could be longer as I work from home a lot and there is no need for it to timeout in 15 minutes. Here's the info on Security policies.