Okay, picture a planet so massive, a black hole has formed in its center. The minimum mass of a black hole is 22 micrograms, or the Planck mass, and whatever the core is made of, it needs to be compressed until there’s a point where the escape velocity is at least the speed of light. If we go with a terrestrial planet like Earth, how massive would the planet need to be for a black hole to form at its core from the sheer pressure?
Your planet needs to be about 722500 times more massive than Earth for its core to undergo collapse into a black hole.
Leaving aside the small detail that at this point your "planet" would look and behave like a star larger than the Sun, because it would be a star larger than the Sun, what will happen shortly afterward is that the rest of it will disappear in the black hole as well, making it bigger. I can't say how long exactly that would take, but the "it's a thing with a black hole at the center" would likely last no more than a couple of seconds, after which there would be just simply a black hole with the mass of thing that was there before it.
A natural occurring black hole that comes into existence due to mass collapsing onto itself must have more mass than the Tolman–Oppenheimer–Volkoff limit, which has been estimated to be around 2.17 solar masses.
There are no planets that are more massive than stars. At around 13 times the mass of Jupiter (in other words, at around 4,134 times the mass of Earth) a planet would be fusing stuff at its core to become a brown dwarf. That would still be way less than a solar mass, but that would also no longer be a planet.