The Automatic LiveUpdates are run about every hour. When you manually run it, it may find the updates that would otherwise be found in the next automatic run, but they get downloaded early because of your interaction. Pulse updates run every 5 to 10 minutes to get any absolutely critical definitions for zero day infections to the users as quickly as possible.
There is no reason to panick about the LiveUpdate schedule. If Norton thought it was necessary to run more often, it would design things that way.