Thanks for using Azure.
There is currently a limit in extending notification past 7 days. Here is an SO post explaining why the limit exists:
"Unfortunately, this restriction isn't quite as arbitrary as it seems. It's coming from an underlying system that Notification Hub uses to park those scheduled notifications that forces that limit, and there is no way to increase that limit.
So we aren't able to pass along any kind of increase to customers. If you scheduled for longer than that, I believe Notification Hubs would reject the request."
Hope that helps.
Best,
Grace