Higher education management. Did a PhD (in a technical subject) then two years of post-doctoral research before deciding to change career.
I think it's been very helpful. I learnt to: tackle long-term problems; work through difficulties; teach students; manage upwards and talk to senior colleagues; manage my own time and expectations; network at conferences; deal with being a woman in a male-dominated field; improve my programming, analytical and communication skills; cope with failure; understand my strengths and weaknesses better. Some of this I might have learnt through a job instead, but I would probably not be as self-reliant or self-aware.
For my career development, I think that I am taken a bit more seriously by my academic colleagues than if I just had a bachelor or master's degree. It definitely means that I understand their perspective on issues such as research funding and performance management much better, and this helps me when trying to persuade them into a course of action. I certainly didn't start the PhD intending to follow this career path, but I am very happy with how things have turned out.
I worked full-time on the PhD but combined it with teaching during terms. I think how long you need depends a lot on the topic. If it's lab based then it might involve long hours, if mathematics then you need shorter but very focused sessions, if archaeology then you may need to work on a dig, and so on.
Over the past few years I have looked through hundreds of CVs for admin roles, some of which come from PhD candidates. What I would say is that you can't expect to get a higher level of role simply because you have the PhD, but instead have to demonstrate very clearly that you have developed high-quality transferable skills as a result of doing it. Unless you are applying for an academic role, experience and skills are more important than qualifications and research papers. Doing a PhD should give the opportunity to improve a wide variety of skills, but you need to be able to show that you have made the most of that opportunity.