You don’t ignite the sacrifice, you place it on a pyre.

Those can burn hot when well constructed. Not quite the kind of heat a modern crematorium can produce, so it is slower. But it wouldn’t have been a full day of burning.

Cooking can take longer than burning. If you threw your steak directly into fire, it would be inedible in the same amount of time it would be medium rare on the grill above the fire (as a rough example, don’t expect precision here), and burnt into a brick in maybe fifteen minutes at most. I’ve lost meat in just coals before, and that’s about all it took, so an open fire would likely be even faster.

Waaay back in the day, to the best I’ve ever read, most sacrifices that were burnt weren’t single sacrifices. This means the fires were also bigger, more intense, than what you might have in your home fireplace. So, once the sacrifice was on the heat, it would ignite relatively quickly. Then, you’ve got fats rendering and burning, which burns pretty damn hot; hot enough that you’d only need an hour or two for the bones to fragment.

Think about it (or look it up if you have a strong stomach), people and animals caught in house fires aren’t in them for massive amounts of time, but they’re essentially carbonized well down towards the bones, and sometimes the bones are “falling apart” (there’s fancy terms for that, but I can’t be arsed to pull them from memory) in the time it takes for the structure to collapse.

Anyway, the rules of sacrifice really varied. In some cases, they weren’t actually burnt, they were cooked. It was the taking of the life that was the sacrifice, so burning wasn’t always part of it. Iirc, it was mostly sun, fire, and similar gods that fire sacrifice to destruction weres the norm. But general purpose sacrificed animals were sometimes cooked and eaten. It really varied a lot over the millennia across the world.

One aspect was though, the burning of the sacrifice was so that it could “rise” to the god/s. A form of transubstantiation, destroying the earthly form and sending it to the divine in its constituent essence. In other aspects, the fire was the god or gods consuming the sacrifice.

Fwiw, if you stack the pyre right, with enough fuel, a human body is reduced to ask and bone fragments in maybe six to eight hours. Something with less mass (a lamb as an example) will be faster.

Plus, some of the really big sacrifices were done en masse in huge fires. Literally tons of wood, often resinous woods that burn hot enough to damage stone at that scale. Can’t recall where, but there’s sacrifice spots that had stone show some melting, which is fucking hot.

To sum up, I guess the answer is that it depends on when and where the sacrifice happened, and why.