How much of the jejunum is bypassed during gastric bypass?
One of the most used surgical technique is Roux en-Y where the small intestine is divided approximately 40-50 cm below the lower stomach outlet and is re-arranged into a Y-configuration.

The portion of intestine (jejunum) from the upper stomach is called the "Roux limb". It has a total length of 80 - 150 cm.

So the Roux en-Y technique "bypasses" only the duodenum and a small portion of jejunum - no more than 50 cm, that's about 7-10% of total length of small intestine.

"Total gastrectomy with Roux-en-Y" by ignis - Own work. licensed CC BY-SA 3.0 via Wikimedia Commons.

Reference: Wikipedia contributors, "Gastric bypass surgery," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Gastric_bypass_surgery&amp;oldid=606081291 (accessed June 27, 2014).