That was the question I was asked recently by a colleague who had been on a working tour of the Far East. He had asked a significant number of first and middle managers to estimate how long they could take in any week to focus on their personal and professional development, and 30 minutes seemed to be the majority answer.
He was surprised at my answer:
“Do nothing but think about what it is you’re doing, and how effectively you’re doing it”
In short, use the time for reflection and don’t feel guilty about doing this as it could be the most important 30 minutes in your week.