This is a very macabre way to ask this question.
It would be less so, perhaps, to say, “How long until a man is naught but bones?”
Or nowadays we’d say, “How long is the decomposition process?”
But Hamlet has rotting on the brain (“There is something rotten in the state of Denmark.”) and, perhaps, given how recently he’s seen his father’s ghost, he might also be wondering what his dad will look like the next time he sees him.