Pjpic, General Relativity breaks down at scales so small that gravity becomes strong enough to induce quantum effects.
An estimate of this scale can be made as follows. For a particle of mass m there is a distance associated with its quantum effects known as the Compton wavelength, h/mc. If you try to confine a particle within a region of this size, pair production will occur and the single-particle description will no longer suffice. The distance associated with the gravitational effects of the particle is the Schwarzschild radius, 2Gm/c2. Breakdown occurs when these two distances are comparable, h/mc ~ Gm/c2. This happens at a mass m ~ √(hc/G) known as the Planck mass, and a distance r ~ √(hG/c3) known as the Planck distance.