Nexus 7 Sleep timelimited to 10 minutes?
The time you can set before the screen locks can be limited by a device policy, just like how a policy can enforce a particular security level for the screen lock (e.g. you must use a password). You might have a device policy if you use a corporate VPN or an Exchange account.

In this case, to increase the timeout or disable the lock screen completely, you'll have to ask your corporate IT to remove the policy, which will also remove your access to the work resources (VPN or Exchange server).