It really depends on the wheel in front of me. Most of the time I am reluctant to get closer than 1/2 a bike length. When behind a consistent, knowledgeable wheel I am comfortable within 6 inches or so. I am not really fond of riding close in large lines. There's always someone in the line who is inconsistent and that just comes down the line at you. Especially on my clubs' B ride. It can get scary.