If you listen to the 911 that I posted earlier, the gunshot isn't until the 50 second part of the video.
And the people who called said the reason they were calling was because there was a man outside yelling for help.
Which means, Martin was on top of Zimmerman, beating him for probably close to 2 minutes (assuming the 911 callers called immediately and didn't spend some time rubbernecking outside their window.).
Dismiss 2 minutes as nothing, but count it out in your head. Then imagine you've got a man on top of you, beating you that entire time while your pleas for help go unanswered, and the person assaulting you doesn't let up.