If this was record breakers in perfect conditions then one per minute is fine but in reality I think 4 per hour is about right, any more is a bonus. It's the case of want happens if? Back boxes could need refitting, short cut cables, lugs not fitting correctly loads of things can go wrong and takes extra time to sort out. If your pricing this up for yourself I would think 4 an hour (with travelling in between rooms) if your prepared with all the tools with you, sleeving, back boxes, lug screws and an extra socket incase you crack/damage one whilst going round. My works give me ten minutes per socket change, sometimes I do it, sometimes I don't. Each one is different but always nice to get an easy change of sockets.