I started a new female mage and tested the level one bless three times, it lasted right at 20 seconds. I used the stopwatch on my phone. Hope this helps.
thanks!
let's say you are within 1 second of error
19~21/15 = 1.3~1.4, which implies that 1 time unit is 1.3~1.4 seconds, or, 3 time unit is 4 seconds.