Mi Band 24/7 during around 3 months now. I already charged it twice during this time, btw.
In the beginning I checked my steps rather often, but recently I do not open the mi fit app that regular.
Therefore I was interested how long can mi band work without synchronization. If you are also interested, this is the answer. The official one:
Mi band can record the data continuously, but it could only keep these data for 7 days.
If it exceeds 7 days, the data would be erased automatically.