Ummm.... That's the art of animation.

You need to study motion.

Stand in the mirror and throw a punch. It probably only took somewhere around a quarter of a second or less. Watch a DVD that shows someone throwing a punch and watch the punch one frame at a time.

The cool think about keyframe animating is that it's really easy to slide keyframes around to adjust the timing. Using the dope sheet or the graph editor you can move keyframes and adjust their values easily.

Try some simple projects just to get use to keyframing and using the animation editors (graph, dope). Have fun!