My understanding is that photons would be converted into pure energy and cease to have the wave-particle properties associated with a photon once it becomes integrated into a black hole's singularity. Light is a wave which, to all observers, appears to travel at light-speed (c). Alhough light has "particle" properties, it radiates in three dimensions as a spherical wave through vacuums as well as throguh translucent media, and its velocity cannot be dissipated (only its direction, frequency, and amplitude can be altered). So it is best thought of as a wave.
I'm not an expert on the subject, but I don't believe the light collected by a black hole adds significantly to its mass. SUper black holes are thought to be at the center of all spiral and cluster galaxies, so you can imagine that, in being surrounded by billions of stars, its light intake would be colossal. Besides, as a mathematical consequence of the Lorentz Transformation, light cannot have mass because it travels at the speed of light, which would render it infinitely massive. Thus the concept of light having mass is only useful in specific applications but is not generally true, as we would assign mass to a true particle.
You asked how much matter it would take to retain some amount of lumens. I believe once an event horizon is established, there's no limit as to how much light a black hole can retain. Any light that enters is not escaping, regardless of quantity. It doesn't max out and start overflowing with light; it's a one-way door with nothing but singularity on the other side. So the answer would be however much matter is required to form a black hole.