All of it.
Which is where writing by the seat of your pants can make it interesting to juggle. Because when Jack's story catches on fire and goes on and on and on, it can be so very easy to forget to leave his POV.
This can be interesting when Jill and Sophia and Mortimer are also major POV characters, raising questions of how long POV sections should be. Especially interesting when they have some down time in their stories, which makes them start to seem less important by not having time on stage. And there are timing issues, to ensure that a reader has some chance of working out what comes before and what comes after.
Even more so when you use bit characters to illuminate the story from odd angles and then drop them. You don't want to drag one in by the ears, to illuminate something the main character could have seen, but you don't want to lose the structure. Especially if POV changes will pick up later. Doubly so if you are near the beginning. It's a lot easier to drop POV switches than to pick them up. The reader is less likely to ponder that he hasn't shifted from one (or two, or three) POVs in a long while than he is to be jolted by wondering who the devil is telling this part. Provided, of course, you don't start too many POVs up front, leaving the reader bewildered and not really engaged in any.