According to the manufacturer, the fitbit Charge 3 should last up to 7 days. However, I recommend waiting for the first reviews as soon as the product is available to confirm this.
For me, that was the most striking and best thing about this tracker: the battery had already lasted 11 (eleven!) days. But then, unfortunately, it was over: right after the 30-day return period had expired, my Charge3 gave up the ghost. Display black, no sign of life...