Fitbit trackers will sync to your Fitbit account every 15 minutes, as long as they are within 15 feet of a computer with a plugged in base station (for Classic and Ultra trackers) or a wireless sync dongle (for Force, Flex, Zip, and One trackers).
Your tracker will only sync if it contains new data since its prior sync. If you are sitting still or not wearing your tracker, a sync will not occur.
You can view the time of your most recent sync by clicking on the gear icon at the top-right of the dashboard. This will show a menu that includes an icon representing your tracker type and an indication of its most recent sync.