For non-critical machines, I'd set the "notification interval" to 0, that way, only one will be sent when it goes down, and one when it comes back OK. You may even want to create a timeperiod entry for this, so it won't even send out notifications 24x7.
For critical devices, you would of course want to be notified every hour or so, so a setting of 60 might be good for notification_interval. You might even want to escalate the problem if the first group of people don't "dissable notifications" or "acknowledge the problem. Read the docs on escalations and try to visualize it where you work.
Example: Joe Tech is notified of a problem once every hour.
He is supposed to "acknowledge" the problem and by doing so, everyone in the contact list will be notified that he acknowleged it and also, no further notifications will go out, until the service changes state.
You may want the procedure to be "Don't disable notifications, and don't acknowledge", so that way, if the problem remains, then the notifications will go out on the 3rd interval due to your escalation settings. That way, the "boss" will know there is a critical device down, and it's been 3 hours now. Joe Tech can in that case, put a comment on the failed service saying "I"m working on the problem, so please give me a few hours to purchase the parts" or whatever.
If you make your settings correct in the escalations.cfg, then you might have this:
Notifaction 1-3 go out to "General techs"
4-6 go out to System Analyst's
7-10 go out to "Da Boss"
There are no further escalations past 10, so just General Techs will get notified from now on, every "notification_interval" or 60 minutes. Somebody has to acknowledge the problem to stop the notifications from going out, or fix the problem.