Sounds like they are using the Notifications app, not Rule Machine. The Notifications app has a "for how long?" option that is supposed to wait so many minutes before sending a notification.
To answer the OP's question, the docs are a little confusing because "events" cannot be "true," but presumably what they mean is something more like "choose for how long the condition caused by the event must be true before a notification is sent." This is straightforward for things like door locks or motion sensors, which I assume is what you mean with "boolean" (sensors that have binary states; this setting in the Notifications app is, of course, always something that evaluates to a boolean value). I'd assume that with continuous values like temperature sensors, if you say something like "< 50 degrees" and "for 2 minutes" would definitely work if a reading of 49 is reached and stays there for at least 2 minutes, and I'd expect but can't say for sure that a reading of 49 for one minute and 48 for one minute after that would do the same (two events but still meeting the requirement; if this isn't the case, then I guess what they meant was more like "choose for how long another event for the same device attribute must not be generated before the notification is sent").
Is your problem that you have a setup like this but get notifications immediately? Or do you get repeated notifications for each new reading below the threshold, every time it is met? I could understand that happening (e.g., 49 for 2 minutes followed by 48 for two minutes would both send notifications because they meet a very simple, naive interpretation of the condition...but probably not what most people want). The former would be quite mysterious. Rule Machien could be used as suggested above to work around either, but we might want to verify with staff that Notifications is behaving as expected, so your exact issue would be good to know.
