Generally about 12" minimum, sometimes more depending on circumstances. I rarely (ever) do pace lines per se, just casual riding with friends on the road and they're as good or better than I at consistency. All my cycling friends are either shorter than me, and/or on smaller frames, and/or riding recumbents, so I rarely get the full benefit of drafting anyway (I'm 6'2" on a 61 frame). The converse is true for them: They say drafting behind me is I'm like riding behind a truck (a slow one) . Every little bit helps, though.