This is a quick chart I made comparing the (average uptime) on shield block, with a warrior's average rage per second gain. The colored DoT's are the bosses swing speed ranging from 3 seconds to 1 second in .2 second intervals.
The method used was
(Swing timer) * (Rage per second) / (Shield Block Cost in rage) = Average up time.
http://i.imgur.com/8XEYf.png

