What are the limits of the Bubblehead charm?

Question

So yes, it creates a bubble of atmosphere around a person's head which also renders them impervious to the material all around them (eq: water). Yes, that's great, but there's so much we don't know about the capabilities of the charm:

1. Is there any explanation as to how that air keeps getting replenished?

2. What would cause it to break?

3. For how long can you keep it on until you feel dizzy or exhausted or have to take it off? Or can you keep in on forever?

4. Can you cast it on literally anything-even an inanimate object- and make it buoyant?

Can anyone provide canon answers for this?

Answer 1

As pointed out in the comments, there is no canon source about the details of the charm. The information we have is:

1. The description:

   "an enormous bubble around his head, which made his features oddly wide and stretched" * - (GF26)*

2. Who used it: in GF26 it was used by Cedric and Fleur underwater, in the OP30 it is used by Hogwarts students to avoid the smell of the twins' Dungbombs and Stink Pellets;
3. Quality: it ensures

   "a supply of fresh clean air" * - (OP30)*

4. Scorpius and Albus used an Engorgement Charm on Cedric, who floated away (CC2.20).

Based on only this, one has to make some speculations:

1. How does the air get replenished - First, I thought that it would simply filter the oxygen out of the water, like the respiratory system of fishes. The characterization in OP30 seems to invalidate this hypothesis. It really seems a supply that is independent of the immediate surrounding. In this sense, there are two options. Either the Charm conjures up a bubble that seems of the size of your face but in reality is much larger, implying in some way an Undetectable Extension Charm. Or it simply works like Aguamenti, and outright creates the oxygen or supplies it from an unknown source.
2. What would cause it to break - although we have no explicit instance of this happening nor do we know the fabric of which the bubble is made of, it could be that the grindylows broke the bubble when they attacked Fleur. As to spells, I would think the Severing Charm would do the job;
3. For how long can you keep it on - minimum an hour and one minute underwater, minimum a corridor walk over the water;
4. Can you cast it on literally anything-even an inanimate object- and make it buoyant - we have no idea about the spell being applied to inanimate objects.