A. Actually we don't really change seats that much, but we do change 'drivers' so each of us takes turns to type in stuff, but again this isn't based on any specific guideline.

B. Yes, it is slightly more taxing than working solo, but the advantage is that you feel more productive and better about yourself because you've completed a lot of stuff in a day.

Just to illustrate this - we've done four sprints of one-week each so far, and in every sprint we've achieved more than we had planned to do (even though are targets are quite ambitious).

A) It depends on your dynamic. I've found that with people I don't know that I tend to switch very frequently (10 minutes) whereas with my long time pairing partner we can go much longer (60 - 90 minutes) without switching.

B) Yes! It is definitely more taxing. That's why there is the idea of the "Eight Hour Burn." Pairing takes it out of you.

http://www.c2.com/cgi/wiki?EightHourBurn