Default settings are to sync 1 minute after a change is made, and every hour thereafter. If you are making changes on the Mac, it will sync after a minute, and the only factor determining whether the iPod gets the change in a hurry will be your usage pattern on the iPod.
It is possible to change the settings with a bit of work. Ken describes how to do it for the Mac in this post:
http://forums.omnigroup.com/showpost...34&postcount=2
To change it on an iOS device, you need to click on a special link (while using the iOS device for browsing). I put a handful of different values on a page here:
http://dl.dropbox.com/u/971671/OmniFocusSync.html