I don't know about other guys, but once i've started to get interested in someone, my interest doesn't go away for months even years!!!
the tough part about guys is not in how fast they lose interest, because they generally don't unless the girl does something to (mess it up).. but.. the hard part is to really spark that interest in a guy!!!
the opposite can be said about a girl... it's quite easy to spark that interest, and it happens rather quickly... **~**(In a Flash!!!)**~**
However, not only can the guy really mess it up.... but the thing with women is that a guy really needs to maintain that interest!!! women get distracted, and lose interest quicker than Monica can go down on her knees!!!
If a guy doesn't keep in touch, it's often frustrating because you really know how interested you were in him.. and you also know that your interest will start to fade away!!! so the one thing you hope for is for him to call back!!! SOON!!!
because you know more than anyone else, that if you don't lose interest in him by the end of the week, then it'll be that other hottie at the bar that clears your mind of his face quicker than ole' Bill can say "I did not have sex with... oh wait.. i'm here for that parking ticket amen't I?"..