What do monitoring tools measure?
These tools keep track of the server uptime.
This value shows how long a monitored web page was or wasn’t available during a 24-hour long period.
How do they work?
They regularly download files from the URLs of the sites that they monitor. The frequency of the downloads vary. Some apps download files once every hour while others do it 60 times in an hour.
For example, if monitoring software checks the contents of a website in 15-minute long intervals, it will try to download files from the site’s URLs every time 15 minutes has passed. And if it encounters an error or a redirect while downloading the files it will send a notification to its users.
Monitoring tools can only notify users about errors that happened during the downloads. So if a website is unavailable for 14 minutes, but there is no download in that period the software won’t register the error.
And if the same site is down for 29 minutes and the app only tries to reach it once during that period it will notify the user about 15-minute long span of unavailability.
Therefore more frequent monitoring leads to better results.
How often should you monitor your site?
I believe, for the best effect you should monitor your website in 5-minute long intervals and at these times you should check the availability of every important page on your site.
This frequency won’t affect your site’s performance in any noticeable way, but will it will allow you to learn about every important error or slowdown.