I find it very inconsistent.
Sometimes when i get near my phone after a run, i can see on my watch the phone connects and it starts transferring immediately.
Sometimes it takes several minutes before it will start syncing after the phone connects.
Sometimes I have to open the Garmin Connect app for it to start syncing after the phone has been connected for a while.
Sometimes it doesn't sync at all and no combination of opening the Garmin Connect app, restarting either device, bluetooth, etc will help.
What's the secret?