It really depends on both research field, your project, and how dedicated you are. For instance, I've found that psychology research is less time-intensive than, say, cellular biology research. Some procedures in cell bio simply requires you to be there for hours, whereas psych research may be more regular (not that I've ever done psych research). Clinical research, where you interview patients, may also be pretty time-consuming, but at least it's a set schedule. If you want a bigger project, expect to take more responsibility for running your own gels or cell cultures, than if you just help a post-doc with his/her project.
I have friends who spends 20 hours a week at the lab = never see him outside of class. I also have friends who do 7 a week. Between 10 to 15 hours seem to be the normal amount.