Thottbot says it is a 10% proc chance with no cool downs so it oprocs pretty often.
The shiffar's Nexus-Horn is 20% chance proc but only on crits keep this in mind, both are really nice i would say for dps quag's eye will provide more dps but when you want to beef up crit for a longer boss fight the horn is awesome. Generaly you will always use lightning cap, but sometimes for trash it is fine just to go with shiffar horn and eye they are great trinkets.
These 2 trinkets even though they are blue more or less are only replaced once you have access too:
http://thottbot.com/i28789
http://thottbot.com/i30626
http://thottbot.com/i32483
I tend to prefer proc based trinkets so I am more partial to the sexatant and mags eye, untill you get some of those hang on to your horn and quag