3 and a half years is the ideal time to finish. Universities aim to get you graduated in 3.5 years and tend to be unhappy if you take more than 4 years unless you have a very solid reason. Sometimes you run into trouble in your degree eg. equipment breakdown, illness, experiments not working out, running out of funding etc and this will all contribute to you finishing later.

I think, to have the best chance of graduating in 3.5 years, you should ensure your project has decent funding, your supervisor is able to provide you sufficient support, work very hard (like 10 hours a day, and going in on some weekends and during public holidays), be as productive as possible, always plan ahead and set goals, ensure you have the required equipments/reagents/support way before you intend to start your experiment and so forth. Always communicate with your supervisor to get feedback on your progress and to express any uncertainties or concerns and work together to solve the problem.