In theory it is possible to complete a PhD in two years (i.e. it is the minimum lenght of time you can be registered for and still get a PhD). But you'll have to work very very hard. You might be able to talk to your prospective supervisour about the possiblity of doing in 2 years - especially if you are self funded. In reality - PhD students are cheap labour and most supervisours like you to stay on as long as possible and do as much work as possible....