If the magnitude of the electric field at a point in air surpasses about 3x10^6 N/C, air will begin to conduct electricity. This is because a field this strong accelerates any free electrons that happen to be around so violently that they crash into nearby air molecules with enough energy to ionize them, which creates more free electrons, etc. This creates a valance of ionization that frees a huge number of electrons for conduction. What is the maximum charge that you could put on the surface of a plastic sphere 1 cm in diameter without causing the air to conduct electric?