This guy stares at me for a long time on occasion, but the usual thing he does is look at me when I'm not looking. I can tell he doesn't want me to see him looking because he pretends to look at something else when I catch him.
My friends have noticed him staring at me for a long period of time when I wasn't looking as well.
I can't really think of a reason why a guy would repeatedly look at a girl if he didn't have some kind of interest in her. I know I only do that with guys I'm interested in.