There is potentially more free time during your PhD years than during any other time of your life after college. You can choose to take advantage of this and do things like pursue outside interests, start a family, etc. Or you can decide that you want to work your butt off 7 days a week, 12 hours a day. There are some people who do this because they want to finish your PhD in 4 years instead of 5 or 6. There are others who do this because they really love their work so much that they don't see it as work or have relatively few outside interests. There are a few people who are watching you who want you to work hard, for different reasons. Your PI will want you to work hard so that he or she can bask in the glory of your achievements. Your MSTP will waht you to work hard so that it seems to applicants and to the NIH that the students finish their program quickly. However, you have to decide whether it's more important to please these people, or to have a life outside of work. Also, your time in the lab is determined to a large extent by how efficient you are, i.e. how much time you spend on SDN vs. pipetting, or whatever you do to make progress in your work.