I suggest starting your research with understanding:

Penrose diagram – Wikipedia

In a nut shell, matter falls to the singularity in a finite amount of time. For the most part if you treat the matter as if it fell at the speed of light relative to the outside observer, you’ll be correct within an order of magnitude. Beyond that it gets tricky. All I can say with 100% certainty is the matter reaches the singularity before the black hole decays from Hawking radiation.

Really what is interest inside the black hole, is how it appears as you fall in. And for that you never actually have a sudden change of crossing the event horizon. From your perspective it is always somewhere in your future, right up util you hit t=0.1s, by which point ever atom of your body has been destroyed.

An external observer sees, the you falling in forever. But just as when we look at distant stars, the observer knows he is looking at the past. The only thing is, from that external observer it is never too late for you to have turned on your rocket engines. It is a calculation that tells them you are at the singularity, not observation. If they calculated with no acceleration, and you used your rockets they could see those activate and you come flying away from the black hole, long after they assumed you were dead.

Matter does not slow down as it approaches the black hole. It

appearsto, fromourperspective. Spacetime itself curves, giving the illusion of slowing. In its reference frame, it sees us speed up. That is relativity.In its perspective it takes

eternityto reach the center.Black holes are no more hollow than any other region of space. The Schwarzschild radius is simply the radius at which light cannot escape because spacetime curves back inward. Infalling matter still occupies the radius when it consumes, just as space is or is not occupied by matter to whatever degree matter is there.

Because black holes are a manifestation of gravity. And gravity causes the heaviest objects to sink to the center of any gravitational well. Moreover, this heaviness creates pressure and pressure creates heat. If anything a black hole should be unbelievably dense and extremely hot. Yet we really do not know the state matter becomes when it enters a black hole.

It may become something that cannot exist outside a black hole. Or it transits as energy thru some other outlet in another universe.

From our point of view this matter will never reach the event horizon (to say nothing about the center), so some people claim that black holes are really hollow. Strictly speaking, for an outside observer the event horizon will never form (at least classically).

What always puzzled me is that everybody seems to forget that there have been stuff inside the (future) event horizon

beforethe whole thing turned into a black hole. So, black hole cannot be really hollow: there must be “almost” a singularity, fuzzy ball or whatever else.A black hole is spherical as far as we can tell. The event horizon, while often pictured as a wall to be passed though, it is actually just a sphere of space where gravity is too strong for even light to escape. As such, the matter and light pulled into the singularity inside said horizon are assumed to be at the very center, compacted to a near infinitely small point (possibly so strong that the mysterious weak force that normally keeps the nucleus of atoms separate from the electron cloud is overcome by a heightened gravity, and the space is literally squeezed out of the atoms.) The Law of Conservation of Mass says that matter cannot be created or destroyed, therefore the matter vacuumed up by the black hole must still be inside.

Hope this helps

To reach the exact center? It takes an infinite amount of time from an outside observers reference frame. However it takes a finite amount of time to reach everywhere except the exact center.