Alerts for down and degraded sites

What you're looking for is alert trigger interval (and alert clear interval). Those aren't settings on the alert rule, but actually on the alert itself. Today, this is also a global setting, so use with care. These intervals specify how many additional poll cycles are required to be in violation (or in a cleared state) before the alert is opened (or cleared). 

Since they are global settings, you have a few options on how to implement (it can be done non-globally, but you should approach it with your eyes open). Take a look and reply back if that's what you're looking for.