To change this setting in Outlook '13:
Send/Receive tab on the ribbon (toolbar) > Send/Receive Groups > Define Send/Receive Groups

Outlook '10:
File menu > Options > Advanced > Send/Receive > Send/Receive button

Outlook '07:
Tools menu > Options > Mail Setup > Send/Receive button and change the scheduled time.

If you have multiple accounts and perhaps do not want to automatically check one or more of them, click the Edit button.

In there you can specify different schedules, include or exclude automatic checking for the various accounts.

But be aware of "pop locking" when you have multiple devices.  See this article.