knockdown on most skills is about the same i believe. ive used the smiting skills and the knockdown is fairly short. they don't stay down for more than a second or two.
im preaty sure knockdowns are 1-2 sec. i dont know if they change with skills tho. it also depends on if you have stone gaugtlets, hope that helps alittle, but you should prob. get a second oppinion incase
edit- well looks like someone made me the second oppinion while typing
Gale is another skill with a duration listed in the effect. (3 seconds)
I have a running theory that knockdown skills do not have a standard duration and vary from skill to skill. 1-2 seconds is where most fall unless they are noted otherwise. That being said Signet of Judgement is an elite, and as such might have a longer then normal knockdown time. I havent been able to confirm this due to its unavailability during beta weekends for me to test. (I have horrible luck with exotic charm vendor, no lie)
It would be nice if Anet would list the duration of each skills knockdown time within the skills descriptor.
All the skills I've tested had knockdowns of 2 seconds except gale and backbreaker. Stonefist Gauntlets add 1 second to the 2 second ones, didn't test with gale, does not help backbreaker.
The thing about knockdown times that makes them tricky to measure, is that you have to time them from the moment you are hit to the moment you can use skills again, which is a little bit after standing up.
Signet of Judgement seems to me to have about the same knockdown duration as most other knockdown skills(excepting Gale and Backbreaker) which seems to be about 2 seconds. Signet of Judgement is not elite for the duration of knockdown(like Backbreaker is), but rather for the reason that it does AoE holy damage as well as knockdown. It's also a signet, immune to many of the interrupt skills and requires 0 energy to use.
For people who want to try it out, it is captureable in the Black Curtain- there's a level 28 monk boss who uses it.... just make sure you use your capture signet after you stand up rather than while he's casting it- otherwise you'll likely get knocked down and have your signet of capture interrupted.