When you are working as a research assistant (and also as a teaching assistant) then you are working on your research. The professor is a collaborator and an adviser, but in the end, its your work. He can give you advice, but in the end, you have to do the heavy lifting, because the prof is busy doing his or her stuff.
No one keeps a time clock, people don't monitor what you are doing. This is in part because it's impossible. If you see me drinking coffee in a coffee house, then how do you know what I'm thinking about. You can't. No one can.
The other thing is that you usually *wish* you had more time. You go to the library, start reading, and when you look at the clock, you suddenly realize that five hours have passed, and you still have a stack of thirty papers that you have to read. It's amazing how quickly time flies when you are doing something that you like to do.