I can only speak to my industry (public accounting) in my location (Texas, US), but there are a line of qualified candidates ready to fill my seat should I choose to quit. Or should I choose to ask for a leave of absence.

Note: this comment is just about my experiences. I'm not trying to make a broad statement about all companies.

I think it depends. A lot of organizations would rather terminate the employee and find a replacement than wait for their leave to end.

Alternatively, many will replace the employee without terminating them; after their leave, they'll re-hire them if the position is open (aka they're hiring), otherwise let them go.