Let's see....at my University, the average time to earning a Ph.D. in a hard science was 5.5 yrs (back in the early to mid-2000s). I knew of labs that would let their students graduate guaranteed in 4 yrs, and some that held their students for 7, or even 10 years. Even one case of the principle investigator (PI) convicted of child molestation, and his entire floor lab was shut down (>40 people are laid off). Students in his lab lost years, and started from scratch in another lab-think they were angry? Forget the letters that the NIH sent around to the PIs, telling them that their next distribution was going to be cut 10%. Post docs lost their jobs over those letters. There is NO stability, no matter how smart or good you are. You will eventually lose your grants, I've seen it happen to the smartest people on the planet. If it can happen to them, it CAN and WILL happen to you. What is the RO1 funding score cut-off now?
There is no compulsion making a PI graduate you. At my institute, we had 2 UK post-docs. They said there was a maximum time to graduate (I think it was 4 yrs), but most people finish in 3 yrs. The average age of a graduating PhD at my institution (top 25 research institution in the US), 32. Average age in the UK...24. Also, when I was interviewing for post-docs, one of the PIs said that the EU post-docs have their salary+travel money+supply money provided by the EU government, while US post-docs do not...I was literally asked, "Why should I take you, with no government funding, over a European?" He did offer me a spot in the lab though...despite him preferentially taking EU post-docs.
Maybe things have changed, but from what I can tell (and have been told from collegaues), it has gotten much worse. Everytime the government fights another war, there is less money made available for research grants. Companies will hire H1 visa scientists (who I found to be excellent), and pay them less and work them more. Just the way it is. Ask why are so few US citizens earning doctorates in science? Do you think education is a way out of 3rd world poverty?
Sorry for rambling, but I strongly urge you to speak to the shock troops/boots-on-the ground in the lab, and get the real scoop. Always ask if the PI is planning on staying or leaving. Even inquire about sabbitical-you might be all alone for a year. We had a saying, "Your PI is either 1) abusive, 2) neglectful, or both!" And, "Do you know how to tell when you are ready to graduate? How? When you want to literally want to punch your PI in the face every time you see them." Sad, but true.