alert for free disk space

currently, I am evaluating NFA 8.6 but I can't receive disk alert email by creating a large file under NFA home directory and get confusion on disk alert setting. Thus, I have the following questions. Pls kindly help. Anyway, the "test mail" is work through GUI

~ is it a instant alert (trigger a alert once reached a specified threshold)

~ if it's not instant alert, what's the interval of the checking ? Can I change the interval ?

~ is it one time alert ? (it doesn't continually fire alert even free disk space is still below the specified threshold)

~ what's the condition of disk space? (based on partition, whole disk, mysql database or directory)