Answer
Zero.
Work Step by Step
We know that the electric field is perpendicular to the equipotential surface. Thus, the work done on a charge to move it from one location to another on the equipotential surface is zero. Put another way, the potential difference from one point to another on the equipotential surface is zero and this implies that the work done on a charge to move it between the two points is zero as well (since $W=q \Delta V$).
