From when you can see the data of a device in grafana

What is the command to check from how many days data flowing in grafana of a particular device?

have you managed to configure a datasource and can visualize your data in a panel? If so, I imagine extending the time-period for your query, or using the time-picker button if you have incorporated a macro or global variable like $__timeFrom. You should be able to see how far back the data appears, perhaps?

That, or you could shell into your mysql server and query the data directly?