http://www.twitch.tv/swingflip2014/c/6820713

This is the first legit video recorded any% of Gears of War 3.

The timer starts when the difficulty is selected and the timer finishes when the last cut scene displays after you defeat tempest.

It's pretty fun to run. Bit lengthy and came across 3-4 glitches along the way which caused me to run around aimlessly to hit triggers or have to restart at last checkpoint.

I would say 4 hours would be the target time for future runs.