In my industry, it is very common. It is generally accepted that a large part of senior staff's time is reviewing the work of junior staff to make the work better. A lot of that requires teaching junior staff how to perform the work correctly.

Depends on the career. As an engineer I really wish we'd quit our decentralized bullshit and just form a guild or union so that after university you join an official apprenticeship rather than find a job looking for people without experience willing to train. The whole x years of experience is often really asking how much mentorship do you need and are you able to lead projects.

Between interns and junior engineers, I'd say I am doing about 0.5 to 1 day a week of 1 on 1 work with all of them. Sometimes it's direct problem solving and other times it's going over topics they are interested in. The last few weeks have been on development processes and workflows, time management and getting things done, presentations and soft skills. I even helped one work on interviewing.

If I can train them to be amazing at their next job chances are they stick around longer and do great things here.