How often does an autopilot update control surfaces' positions?
So often that to any observer (visual or tactile) it appears continuous. 

I say this as someone who has 2500 hours operating a transport category airplane. The flight director provides continuous guidance that the autopilot continuously follows. When holding the control column with autopilot on (below 10k feet you hold the controls even with autopilot engaged) you would interpret it as continuous input (the autopilot moves the controls just as if the other pilot were flying).