1) 3 years (although the plan is to make this 4 years, ie. 3 yrs research + 1 year writing up - I was the first student in my department to be put on this new pathway)
2) Between 3.5 to 4 years - most people are under financial pressure to finish as soon as possible as they are only funded for 3 years - this is why they are trying to shift to the four year programme
3) There is an official deadline - I'm pretty sure it is 5 years but can be up to 6 years if the student has extenuating circumstances (illness etc.)
Those are the figures for full time - double them for part time students.
PS. I am in the sciences if that makes any difference!!
Also 'completion' is a loose word and it depends on when you think completed is. I didnt really think about it until I started my 3rd year but completion can refer to many steps. Is it when you submit? Finished your Viva? Finished your corrections after your viva? Or when you are awarded your degree?