Diablo® III

Smoke screen lasts for 2 seconds by default. Lingering Fog rune increases the duration to 3 seconds. I just noticed that the rune stopped working (which caused me many painful deaths, on inferno) I timed it with a stopwatch many times and smoke screen runs out at exactly 2 second mark.

Yes, I have the rune equipped and it says that in the smoke screen skill's tooltip.
Other skills that I'm using are: Caltrops, preparation, hungering arrow, elemental arrow, companion, stead aim, sharpshooter and archery.

Read the hotfix notes.

Source: http://us.battle.net/d3/en/blog/5825330/Diablo_III_Hotfixes_-_May_Updated_52312-5_23_2012#blog

Classes

• Demon Hunter
• - Active Skills
• Smoke Screen
• Duration reduced from 2 seconds to 1 second (tooltip will still show 2 seconds)
• - Skill Rune - Lingering Fog
• Now increases the duration of Smoke Screen to 1.5 seconds (tooltip will still show 3 seconds)