Sorry for the slow response, was gone for two weeks and I missed your post.
The timeout cannot be adjusted, there's probably a cookie set or a setting saved in the database for when you last viewed "New Content". However, when you are viewing "New Content", you can adjust the period of time you're viewing. Take a look on the left, under the header By time period. You can select "Content I have not read", "New since my last visit", "Past 24 hours", "Past week", "Past 2 weeks", "Past month", "Past 6 months", "Past year":
I assume you were using "New since my last visit". Since you were idle for over an hour, when you refreshed the page the forum considers this a new visit.
With the new forum software that's coming, you'll have much greater control over viewing new content and can setup and save "View new content" queries to your liking (and you can even create several).