If you have auto sync turned on, the iOS versions will sync 1 minute after you make a change, and should sync hourly thereafter (same as the desktop) provided the application is running and can make an Internet connection to the sync server. It will also sync on startup if the last sync was more than an hour ago, I believe.
With auto sync off, it only syncs when you tap the sync button.
To change the sync intervals, see this post:
http://forums.omnigroup.com/showpost...34&postcount=2