Push notifications are sent in real time. It's not necessary for your subscriber to be there on your website when you're sending notifications. Messages are received instantly, so even if your subscriber is on a vacation or login into the browser after a long gap, don’t flood him with notifications from your website. Use Frequency capping to cap the old notifications shown to your push subscriber and to reduce unsubscription rate.

You can also schedule push notifications in advance and can send it at a particular date or time.