It does not expire as stated on various websites. It never expires especially the concentrated type. I think this is based on the process involved in its manufacture and the products that are used as well. This is as stated at http://www.chacha.com/question/can-antifreeze-expire.
You need antifreeze all the time. It is also a water pump lubricant and anti corrosion properties to protect aluminum parts. It also raises the boiling point of your coolant for an added cushion when driving.