The TTL element is optional and some feeds (such as those from the Stack Trilogy) don’t contain it.
When Outlook doesn’t find a TTL, everything defaults to 60 minutes.
I've recently run into a problem with the RSS reader in MS Outlook 2010.
I have been using Outlook for the last 18 months or so, and have subscribed to various RSS feeds for most of that time period, without any problems.
So in this default situation, Outlook will check for updates once per hour.