If your reaction is to the flu shot, symptoms will begin within 6 to 12 hours. Also, the (not likely) side effects of flu shot are soreness/redness at injection site, some general malaise, and some muscle aches. HOWEVER these side effects, if caused by the flu shot, only last 1 to 2 days. Influenza (the real thing) lasts much longer, and the symptoms are much more severe, much more debilitating, and the risk of complications (pneumonia or other secondary infections) are much, much greater. That said, the vaccine takes WEEKS to give you immunity, so anyone getting influenza before that, your shot may not have been protecting you yet.
Getting wordy -- I'll finish up quickly. It is a very individual decision, whether or not to get the flu shot. I get mine, because, with clinicals and the zero absence requirement, getting an illness would be catastrophic. I tell dh to get one because as a teacher, he is mega-exposed.
Oh...a pp mentioned that flu-mist is different. TRUE. It is a nasal spray, and is not the same killed virus that is used for the flu shot. The hospital I had clinicals at last semester didn't let anyone onto the oncology floor if they'd had the FluMist vaccine in the past 7 days.