For how long is a primer usable before the final paint is applied?

I would read the back of the primer can directions, it should be ready to paint over within 48 hours if need be.

Primer is used to prep the wall for the finish coats of paint. There is no rush to finish painting it, I wouldn't recommend waiting to long. Since the primmer is used to give a clean surface to paint upon. After awhile the walls can become in need of being primed again, adding more work and possible cost.

Yes, that should not be a problem as long as the surface is clean and dust free.

I painted for over 30 years. yes you can paint over primer at any time . The purpose of primer is to seal the wall and give the surface a tooth for the new paint to stick to. yes it needs to be clean but more over Dust free. If it has been more than two mouths I would give the wall a light sanding and then a good dusting. Use 150 grit sand paper.

Good painting to you.