I remember my first engineering director telling me he assessed roles by how long it would be before you were really missed. He reckoned in his case it was 5 years, for the chief engineer 2, at my immediate boss’s level 6 months. It had taken 3 months to fill my predecessor’s role and that was about right.
If you felt you were so essential that you couldn’t be out of the office, you weren’t delegating properly.
Obviously engineering research is a bit different from, say, a call centre, but it’s close to what Musk is supposed to be doing.