The xiaomi devices check in about ever 50 minutes. However every now and then they may skip a checkin (I don't know if its just the message getting lost) but they are still online and functioning. My rule of thumb is usually after about 3 misses Its dropped.
That being said I have about 60 devices and they very rarely drop off.
You can use a tool like device watchdog to monitor the last activity and ensure they are checking in frequently.