Peter Gratton's PhD experiences certainly seem to be exceptional. I'm not even sure if it is possible to finish a PhD inside eighteen months in the UK -- in political science at least. Most departments would not want to lose the funding for one thing (and so would surely attempt to refuse early finishers if at all possible) and any students with Research Council funding are funded for three years so it wouldn't make any sense for them to finish early even if they could -- they might as well work on something else in the meantime and finish the thesis off at the three year point.
I am not currently a PhD student but I have many friends that are and it seems that their supervisors expect them to work their thesis over and over several times before they actually commit to a topic or a question firmly, let alone complete the thing. This may be due to badly thought through ideas but it certainly seems that this is seen as being a formative experience that all but the most utterly exceptional students are expected to go through.