Subscribe to push notification with specific channel without expiration date

I want user to subscribe to push notification with specific channel until user unsubscribe or delete the app without the expiration date. Is there a way or method to do so?

If we talk about our SDK:
“subscribe” - mean pub/sub notifications.
“registration” - Push notifications.
You could use “expiration” field (type Date), and set it far in the future.
But keep in mind, that google server (or push servers of another company) can change token in arbitrary time without any notification. Thats why you should provide custom logic for handle such situation.

What is the maximum of the expiration?

I would set it to 100 years in the future and you should be good.