Perhaps you mean the usual notification by email, to tell us a post has been added to a thread which we subscribe to? Normally the system sends these after a few minutes.
However sometimes I get a notification after an extra day or two have gone past. I do not know why.
I seem to remember a few cases where I looked at 'My Subsriptions', and saw that a thread was updated, but I did not remember receiving an email. Again I do not know why.
Notifications at the top of the page, when a subscribed thread gets updated.
It works most of the time, but once in a while the list forgets. It may be on occasions when a system reset occurs, or when the system time is changed. Etc.