In my subject 3-4 years is normal and the deadline is 4 years, but there is someone I know who has taken 6 years to finish.

I took 4yrs and 3mths...as the 4yr deadline was approaching I wrote to my Uni's research degree committee explaining the reasons why I hadn't manage to complete and requesting an extension. I asked for 6mths and they offered me 3mths so I advise asking for more than you think you need in case a) they offer you less, or b) you do need more time than you think (they don't much like 2nd and 3rd requests for extensions!) If you have your supervisor's support (i.e. they can vouch that you have genuine reasons for needing the extension), I'm sure your request will be considered.

it's so nice to read other people's opinions, thank you so much.

I've asked our department about the deadline, and I need to submit the thesis before Oct and have the viva afterwards, a little bit relieved.

I got the funding from the university for 3 years, and our department is good at remindering us our responsibilities. But I was not used to work on my own, and I was too shy to talk to other phd students in our department because of my personality and also the language. So I didn't work as efficiently and as happily as I was in my own country. Now, I have to re-read a lot of articles/papers for the introduciotns and discussions in each chapter in the thesis, and also re-study some points of the statistics by myself, and I just realized that the chapters I've written up were just rubbish, well, not rubbish, but not good at all. That's why I got really stressed.

bty, if a student's viva is failed, does that have any bad effect of the department's RAE(RAT??)