[RPG] How much time passes on the Astral plane relative to the Material plane

From the Pathfinder SRD entry on the Astral Plane:

Timeless: Age, hunger, thirst, afflictions (such as diseases, curses, and poisons), and natural healing don't function in the Astral Plane, though they resume functioning when the traveler leaves the Astral Plane.

The Astral Plane is not Timeless in regards to magic, so you couldn't cheap (cheat?) permanence a Demiplane from there.

However Create Demiplane, Lesser states that:

You must be on the Astral or Ethereal Plane or on a plane that has access to one of those planes (such as the Material Plane) to cast this spell.

If you can find another plane that does feature Timeless magic and has access to the Astral or Ethereal planes (and you could convince your GM that this would work), you would be able to cast one of the forms of Create Demiplane there and thus have your own permanent Demiplane, although as the Timeless feature states it will still be susceptible to dispelling.

Yes, you can use astral projection from planes other than the Material Plane. The Manual of the Planes describes how spells that access transitive planes such as the Astral Plane function on other planes in the section "Moving Among Transitive Planes" (starting on page 45).

Characters generally must use spells or spell-like abilities to access a Transitive Plane. ... the astral projection spell takes you to the Astral Plane ... Such spells should function in any location coexistent with or coterminous to the plane.

It includes various examples of travel between transitive planes on page 46. These examples reinforce that "Material Plane" is a placeholder and that you can use astral projection from anywhere that is connected to the astral plane. Your body will be left behind on the plane you project from.

From the Ethereal Plane to the Astral Plane: You can move to the Astral Plane from almost everywhere on the Ethereal Plane, because the Astral Plane connects to everywhere on the Ethereal Plane. ... If you're using an astral projection spell, your physical body remains in the Ethereal Plane ...

From the Plane of Shadow to the Astral Plane: From the Plane of Shadow, you can use the astral projection spell, leaving your body behind on that plane ...