An infinite amount, if the writer is stubborn and persistent.
Case in point: I now have a novel manuscript that used to be a mid-size novel-in-verse that was once a short novel-in-verse that was once revised to a different gender that was once a picture book poetry manuscript that was once a poem. Got that? Two editors (maybe three) asked for revisions at various points, which lead me to hope for a contract. But it hasn’t happened so far.
Now the novel is out on submissions again, this time in prose, and almost three times the length of the last novel-in-verse rendition.
Why don’t I throw in the towel? Because my agent says he loves it. And more importantly, because my oldest son is the inspiration behind a boy who is passionate about soccer. It’s his turn for a dedication page in his honor.