Find My will display a "Last Known Location" for about 7 days. After that, when you log on to Find My, you will see an Offline message.
If Find My has displayed the location of the phone for 10+ days, this tells me that the location that you see at this time is probably current. The phone is "on", the battery has power, and the phone has an Internet connection.
If you see a message of Offline when you log on using Find My, there are several possibilities for this:
The battery has run down
The phone was powered off
The phone does not have an Internet connection