In Australia it's officially 3 years (with an optional 6 month extension), but out of all other students I knew in grad school there was only one that finished

In Australia it's officially 3 years (with an optional 6 month extension), but out of all other students I knew in grad school there was only one that finished in this time (I'm not sure she ever left her office in that time). I took 7 years, which was not uncommon in my department. What usually happens is that after the three years (+ extension if you're lucky) is up, you don't have a scholarship any more, and for most people this means they have to find a job to support themselves, which leaves little to no time to work on the thesis. I was fortunate enough to get a teaching position at the same university, but even that was very demanding and my progress slowed to a crawl, though I did eventually manage to finish. Universities only get paid if the student completes, so they tend to be pretty flexible. If they kick you out at the three year mark they lose; if there's indications you're likely to complete at some point, they have a chance of getting paid, so there's some flexibility here. I came to the conclusion that ultimately the timeframe doesn't matter - what counts is that you get it done; it takes as long as it takes. A friend of mine was at it for 20 years, but eventually finished. University administration were not exactly pleased with this timeframe and she went through a lot of difficulties because of this, but a remark she made once really stuck to me: "You succeed in spite of the system, not because of it". Caveat: Both of us are Australian citizens; I can see how the limit would be extremely problematic for international students due to visa rules.