An alert triggers when a sensor(s) meet the conditions of an alert.

Using the above example, if a sensor matches the criteria for 5 days the sensor will trigger. However, you're not going to get a new alert unless the conditions are invalidated. Because the temperature was always greater than or equal to 19 degrees the previous alert would just continue until it went out of the alert state.

Another example is if you imagine the condition was "temperature is less than 50 degrees for 30 seconds". You're not going to get a new alert every 30 seconds. Instead you will get one alert when the temperature has been less than 50 degrees for 30 seconds and not get a new one until the temperature has gone above 50 degrees and then again been less than 50 degrees for 30 seconds.

For more information about sensor states see here.