There is no way to answer this question. It seems to get asked a lot on the internet. I suppose it all depends on 1) How productive you are during ‘work time’ 2) How long it takes to gain your ‘data’ to analyse (I think this is the biggie) and 3) How quick your supervisor is at replying when you need an answer.
I am hoping, fingers, toes, legs and arms crossed that I am completed in 3.5 years. At the moment (1.5 years in) that seems realistic. Although I may find that none of my experiments work for the next 6 months, which will mean I will need to carry on my PhD for a little bit longer…
If you are starting out on a PhD, it is good to have a goal for when you finish but equally I think you need to be prepared to be flexible. All PhDs are different. The nature of a PhD is that you are exploring the unknown, delving into the depths of your subject and that takes time (and effort). A good, realistic plan with set targets will help keep you on track but equally some delays will be unavoidable and outside your control. Everyone goes through periods of difficulty and it is the way that you handle the tough times that will speed up the process.
Having a wide range of contacts can help. They can support you when you need to have a moan, but they may also be able to offer you a solution to a problem which appears to be unfixable.
There will always be some people that work 12 hour days and through the night (some choose to). There will always be supervisors that judge you for not working 24/7. Ultimately the PhD is your project and you need to do what you feel is right. Don’t forget, sometimes, supervisors can be right too!