Device tokens can change at anypoint. Anytime your app receives a token, it should store it. Then, whenever a new token is received , compare the new token to the stored token and, if they are different then unsubscribe old token from ethinking push service and subscribe for pushes with new token.

It is good to refresh the push subscriptions on periodic basis from the app. We recommend to do this at least once every month. But do not do this very frequently to avoid overloading the server.