I don't think they do have to wait - I've seen signs office-walls in town talking about 'ruthless' clampers, so that would point to their moving v.quiick to clamp.
In an area like this, I wouldn't leave the car for 5mins for fear that it would be clamped, but I've also accidentally let the ticket on the car expire for nearly an hour without it being seen and clamped.
I think it depends but I wouldn't count on any period of grace - that includes the 10 unpaid minutes on your ticket!