Answer
10 J.
Work Step by Step
When released, the 10 J of potential energy (stored in the system by the work you performed on the electric charge) transforms into 10 J of kinetic energy just as it returns to its starting position.
This situation is analogous to expending 10 J of work to raise a mass to a position where it has 10 J of gravitational PE. When released, it falls back to where it started, and has 10 J of KE.