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.