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