I don't know the number but the limit is the speed of light and expansion of the universe. In the modern phase of the universe a black hole would only be able t

I don't know the number but the limit is the speed of light and expansion of the universe. In the modern phase of the universe a black hole would only be able to eat its own galactic cluster (or maybe supercluster). Hubble expansion means superclusters experience the expansion of spacetime so certainly any matter beyond a black hole's own supercluster will move away from the black hole faster than light and thus never be able to fall into it. In practice a black hole can't even get near that limit because angular momentum can't just vanish... much of the matter would (over a long time) end up in distant orbit around that super super massive black hole and never fall into it. In the early universe spacetime expansion was happening so quickly that it was difficult for large black holes to form at all. If that were not the case most matter in our universe would have collapsed due to gravity and ended up in black holes but we know it did not. Actually if gravity were that strong or expansion were slower/weaker then the universe would never have formed at all and stayed as a singularity. To get formation of super large black holes very early you'd need a knife-blade balancing act of gravity vs expansion to just barely form super super massive black holes without the "permanent singularity, no universe" scenario. We can see billions of stars, galaxies, etc so we know that scenario did not happen. Super super massive black holes are neat objects though. If this one really is as big as they say then it will be one of the last objects left in the universe. It would take trillions and trillions of years for it to evaporate due to Hawking radiation. I wonder if the last life left in the universe will end up gathered around this black hole, surviving on the Hawking Radiation energy gradient?