Hi @striker,
I guess you’re the first user in the forum, who wants to stop receiving notifications 
Just kidding
Reg. your question: you can experiment with “conditional” sending of notifications (make the action conditional). I.e. you can determine if Current User (the chat counter-party) has not been active for some time, then the you send the notification.
You can find the thread reg. “active” users here: How to display only online users on a screen?
Probably you need to adjust this functionality to chat window itself.
This is not a step-by-step instruction, but rather a path to explore
Best regards, Victor.