For us I'd say basic comfort was quick, 20 miles or so on a couple of short rides. What I mean by basic comfort is starting, stopping, breaking with constant verbal communication. It was sometime after 200-300 miles when we began to communicate through the pedals and the need for verbal communication became less important, i.e. we can read each others effort and follow accordingly. Around 500 miles my stoker went to clip-less pedals and stayed clipped in at stops.

I would say there are different levels of comfort too, kicking back on a rail-trail verses city streets or screaming down a mountain hillside are not the same comfort level, it's an on-going process.