I've long since gotten rid of my Blade Ward enchant, but our Prot Paladin is still using it. Here's a link to a 25 man XT Hard Mode kill:
WoW Meter Online - Combatlog Replay
8:53 (533 seconds), 6 procs, 28 seconds of uptime (5%), 4 Parries while Buff was active (1050 average damage).
So, also interesting here. First, I can't really see a way to tell if the buff stacked or not from the WMO, but I'll assume they didn't stack at all (given the low proc rate). If there were 6 procs, and only 4 triggered parries while the buff was up, that means 2 of the procs were up for their full 10 second duration and then fell off without being consumed. So 20 of the 28 seconds of uptime were from those "unused" procs. Futher, this means that the remaining 8 seconds of uptime had 4 parries, an average of 2 second of the Buff being up before a parry occured (and an overall average of 4.6 seconds of the buff being active before a parry occurs).
Seems like the uptime of Blade Ward for paladins is a lot less than for warriors.
[Also, there are a ton of failed XT-Hard attempts in that WMO all with the Blade Ward Paladin MTing. A lot of data.]