In theory, you should be able to sync once every 2 weeks but it's a good idea to do it more regularly if just for motivation reasons. Sometimes there's a delay before previous days show up as the database needs to be updated and compiled.
You should be able to sync only once per 2 weeks, but there is something to watch out for; with activity tracking on and Bluetooth on, but no internet on my phone (while abroad for example), activities will be stored on the Fenix 3 for months, but activity tracking data starts to get removed after about 3 to 4 days. This depends on the settings that Garmin has set for the device, those can differ per model, so it can be more or less. With Bluetooth off on the watch, activity tracking history should be kept for far longer, but to play it safe syncing more often is advisable.
