How long does an old FCM token stay valid for after a new FCM one is issued?

[Orange Testo]

Hello! 

I am looking for documentation/clarification on the time period where a FCM token stays valid for after a new token is issued through getToken. The scenario is app has a FCM token (old token), then app gets a new token manually through getToken call (new token); how long does the old token stay as a valid token in the perspective of Firebase?

Any insights are appreciated, thank you!