Hey Nicolas, how are you?
Based on the time since you asked for this, I guess you already know the answer, but I'm adding it here in hope it can be useful to someone else.
Looking at the Alfresco Sync's source code, I saw the default poll frequency for files create on the server side is 300 seconds (5 minutes).
I've just made a set of tests, and I can confirm that.
For files created on the local disc, the sync process is done just after the file is saved.
So, 5 minutes from Alfresco to local, and immediately from local to Alfresco.
I hope this answer some questions.
See ya.