The best mentor eventually becomes less necessary.

The measure of good mentorship is a declining need for it. If someone still requires you after years, either the teaching has not worked or it has been arranged to keep them dependent. Being needed is pleasant, and it quietly conflicts with the aim, which is to produce someone who can manage without you.

Ask whether the people you help are getting more independent or less. If less, start withholding answers and asking questions instead. Aim to make yourself gradually unnecessary.