More specifically in finance/economics. As I understand it, spending at least 5 years in the program is highly beneficial if one intends to work in academia. However, if aiming for industry positions instead some would argue that it might not add as much value as the forgone salary would justify.
This leads me to my question. Let's consider a person who has completed M.Sc prior to entering a PhD program in US in Finance/Econ or another similar field and is on par with the average candidate in, say T20 school. With hard work and long nights, how quickly could such feasibly expect to complete their PhD?
Furthermore, are there major differences between programs in e.g how fast the coursework can be finished, and between requirements one needs to fill before being allowed to graduate? How about supervisors, i.e. are some less willing to let you graduate ahead of time at the risk of suffering some personal reputational damage if it turns out that the candidate falls short of the standards typically expected from a fresh PhD graduate? Any other factors I fail to consider?
The first thing you need to ask yourself is: do you really want a PhD? At its core a PhD is nothing more and nothing less than "vocational education for researchers". In a PhD program you will learn about and get some experience in doing real research. If you don't want to become a researcher, then what is the value to you? There is the status that some feel comes attached with the title, but is that really worth a couple of years of your life?
Assuming you really want to go through with this, then you need to realize that it will take at least 3 years and probably more. That is a long time, so you need to approach that more like a marathon than as a sprint. So don't go in and start burning the midnight oil, because you will only burn out yourself before you are done with your PhD. This is a very common mistake among PhD students.
The only PhD student I know who made it in about 3 years had a completely different approach. She had a plan that was realistic, and she stuck to it. Making a realistic plan for your thesis is very hard. She managged because she already had experience doing research and was already well versed in her topic. She was also extremely disciplined in how she managed her time, and this ment that while at work she worked very hard, but she would not work in the evenings and not work in the weekends. She was able to work that hard and that focussed for 3 years on end because she did not work in the evenings and in the weekends.
I am not that person. I whish I was that disciplined, but I am not, and it took me 7 years to finish my PhD...