A complicated question about $E=mc^2$ I know this is a little outside the normal question and there may not be a direct answer, but it is an interesting thought experiment.
Starting with a supermassive black hole, if you were able to feed in mass without limit, at what point (total mass) would gravity tear apart all the particles in the singularity to the point that all the mass would be converted to energy? I am assuming that matter is considered energy when it is broken down to its simple building blocks.
 A: Once inside the Schwarzschild horizon, it is immaterial to observers outside whether what went in was "matter" (fermionic stuff with a rest mass) or "pure energy" (bosonic stuff): it all adds to the black hole's mass by the amount  $\sqrt{m_0+(p^2/c^2)}$ where $m_0$  is the rest mass and $p$ its momentum (at infinity). After this, all the body's energy stays inside the horizon. How it interacts with the "singularity" is unknown: General Relativity will presumably be inaccurate near the singularity and one of the reasons for studying quantum gravity is to answer questions like yours.
Mass falling into a black hole can radiate enormous amounts of energy, but this is kinetic energy, rather than rest energy, converted to radiation either as charged mass accelerates (recall the Lamor Formula and its relativistic broadening through the Liénard–Wiechert potential) or as the molecules of an accelerated gas collide and the whole mass becomes fantastically hot. For a spectacular view of this process witness the five thousand light year long jet from the Messier 87 Galaxy.
A: 
I am assuming that matter is considered energy when it is broken down to its simple building blocks.

If that's your criteria for being "considered energy", then at least classically the singularity of a black hole of any mass whatsoever will do this, simply because the gravitational tidal forces diverge to infinity near those types of (curvature) singularities. Roughly, for any material objects, the difference in gravitational effects between its different parts will be arbitrarily large, tearing it apart before some time before reaching the singularity.
What happens near the singularity quantum-mechanically is not really understood at this time.
But there is no rigorous notion for when matter is considered energy simply because energy is a property of physical systems and physical things, whether particles or fields.
