If a device is offline for a period longer than the session time, it will result in data loss - thus have in mind that analytics collection requires a stable internet connection.

Example 1: In my client's store, there is an unstable internet access/electrical supply. It turns off our router for 30 minutes then comes back. Will I lose analytics data?
Answer: If your session time is superior to 30 minutes, then no, you will not lose any data. It's ok!

Example 2: In a second store, we don't have internet access at weekends, due to high traffic of the shared network. Will I lose analytics data?
Answer: Yes, you will lose analytics data on weekends, since the offline time will be superior to your session time.