Despite what the in-game description suggests, you can crit even with Suppressing fire turned on. So, Aim+Suppressing won't prevent criticals. The attack penalty lasts for a base duration of 10 seconds. HIgher levels enemies will be penalized for lesser duration, but I don't know the formula for that. Didn't find it in the wiki either.
You can consult the dragon age
wiki for more and all things dragon age. Also, if you'd like to optimize your archer, make sure to check out the article mentioned by Jeffon above.