Totally depends on what is individual’s situation is. Typically it takes between 6–7 years in American reputed universities. Every university has some guidelines, among them a thesis is must with some publications. This is academic requirements. As long as one satisfies academic requirements university awards PhD. However one may not be doing or contributing to something great. Sometimes people do because of they want to get promoted from lecturer to professor etc. Many time people do because they love investigating or research. It can frustrating at time specially when university stops funding and no support from PhD advisors. If you are lucky all things go smoothly, however many a time one finds themselves in very difficult situations, like I had a change of seven advisors and things did not go well, had to drop out after being candidate a thesis away. It does not happen usually in normal circumstances though. Also, a lot depends on university reputation, funding, advisors support, and most importantly are of interest. Keep a fall back option always in case you need to drop out.
In Europe however it is more structured and there is timeline usually between 3–5 years. Usually difficult to get into PhD programs for international students. If one gets admitted in Scandinavian universities, my impression is that it may be much smoother as compared to American universities. In America if research is not tied with “Money”, it usually does not consider valuable while in Europe this is not the case. By “Money” I meant that research output whether it can be useful in making money somehow. American universities tend to differentiate on this as well. If research project is not bringing money to university in terms of industrial or government funding etc, it may be harder to finish as American universities stops funding and support.
Be careful before you make a decision. One wrong step may cost a LOT!
I hope my answer is helpful.
All the best.
I have answered this question on Quora: https://www.quora.com/q/smg/How-hard-is-a-PhD-3
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