When a file is added to the shared folder, the changes start syncing immediately (due to system peculiarities, sync on Mac OS X 10.6 may be delayed up to 10 minutes). If you change a file inside a shared folder, sync will start after the file is saved and/or closed.
It is also possible to change the period of automatic rescanning of sync folders. You can do it in Advanced application preferences - Rescan interval. Default rescan period equals 600 seconds (10 minutes).