Interesting because I have heard that some institutions are now trying to push people through in 4 for budgetary reasons.
My school funds for 5 years. It is possible to extend beyond that, but you would have to take up some additional duties (like lecturing) and get permission from higher ups.
My school is an international school (probably around top 50 worldwide?)
Some schools are pretty well known for keeping PhDs longer. I think it is a bit unfortunate because the playing field is no longer level. How can someone compete with a person who has had 7 years to do a PhD (and subsequently has multiple first author A's) ... at that point they are more like a junior AP than a PhD ... just my 2 cents.