All of the below information is regarding U.S. universities.
From what I know, it seems that the standard length of time to be funded for a PhD in a STEM field is about 5 or 6 years. (I've personally seen plenty of students get funded for a 6th and final year, for applied math topics.)
So I recently learned of a 10-year funded PhD (one student I know of and have spoken to, but not sure whether it was a program consisting of multiple students). This would be at a Top 5 school in the STEM field.
FWIW, it was an electrical engineering topic.
Is this an exceptional case? Perhaps the project was exceptionally important and the student was funded for 10 years without any issues? Or, is it actually more common than we would think?
(Note that I am not talking about TA'ing for ten years.)
I don't have any data for lower-tier programs, but I'd like to ask the question, just in case there are some interesting / surprising answers.
I am assuming that the research ended up taking longer than anticipated instead of the possibility that the program was known to take ten years to complete. After all, I have never heard of a PhD program that lasts longer than six years.
In the United States, long PhDs (taking more than 10 years) are a minority of STEM PhDs, but not exactly "exceptional" either. See image below, which shows percentage of 2006-10 doctorate recipients taking six years or less and more than ten years to complete their degree (via data collected in the Survey of Earned Doctorates):
(Image via MLA Office of Research blog)
In many US departments, if a student is making progress towards the degree (not just wasting resources), is a contributing member of the department, and funding is available, there is no general reason why the advisor can't continue funding if the degree takes longer than expected. Without funding, the student is much less likely to complete the PhD, which is an undesirable outcome for both the student, the advisor, and the department.
(In some parts of the world, there may be regulations that make it impossible to keep funding a PhD student for that long. Even in the US, some departments may set a time limit on PhDs, and/or on eligibility for some sources of funding.)