But if your culture involves ranking and paying people only relative to each other, I can understand the need to protect yourself. And everybody loses :( The company loses, you lose, the potential mentees lose :(
I like metrics based on how much time the other person has and expects to waste not being able to do anything productive, which strongly suggests you should give him at least two things he can be doing, an obvious second one being reading documentation or otherwise researching necessary stuff he doesn't yet know. After a certain number of hours it's worth an interruption.
And there's the reverse effect that teaching someone else forces you to better learn the subject, so you should gain some extra productivity from this.
Delivered four senior developers in order to accelerate our team's velocity...
that's what my resume says.
Which implies it was only valuable to future companies you worked for. Those that actually believe in mentoring, which seems to be rare based on what others are saying and my own experiences.
I am pretty senior and I like mentoring people. But the incentives are generally the other way: If as an IC I transfer my experience to new people I almost get no credit. The manager of the group gets credit for building a good team but I have to justify my salary compared to the people I mentored. So sometimes I wonder if it would be smarter to keep knowledge to myself so I can look better.