hi loopy searching through for things about vivas and saw your post was unanswered. i took about 7 years (not including 2 years leave of absence ) so may not be the best model in this, (i also did a taught doctorate which was a bit different i guess) as i did very little for long swathes of time due to work commitments. it'll also depend how many years you intend to do it for. i think if you did 10 hours a week solidly and didn't let it get squeezed out by other things, with additional hours in those weeks when for example you collected data or did a lot of analysis, you would get there, but dont' hold me to it. i think if you did 20 hours a week you would probably finish in 4 years. depends how many false starts you have along the way! are you going to be working as well as phd and dcs? i had a lot more success when i gave up work i then devoted 15 horus a a week to it plus a couple of evenings and it went better.