Damien I am sure there are lot of helmets still in use that have fallen off the seat of a their owners bike several times. If a helmet suffers significant damage by being dropped a metre it would not be much use when coming off a bike at some speed.
I have recently discarded some very old helmets that were sitting in a cupboard for at least 20 years. The major deterioration in them was the soft foam.
I think the life of a helmet would depend on several factors:-
1. Quality of the helmet to start with
2. Significant impact
3. Exposure to ultraviolet light (is it left on the bike in the sun all day while at work?), worn in Alice Springs or England?
4. How much it is used, once a week for a few hours, daily for for an hour each way to work?
5. Does the user sweat a lot, is the lining kept clean
6. Has there been a significant improvement in design
There is always the question of how much do you think you head is worth but like all things in life we make decisions about where we draw the line. Most of us have made the decision that we are going to ride a bike, we don't all drive Volvos.
The outside of crash helmets is made from very tough fibreglass like material, we don't see yachts /sailing boats made of fibreglass (which are subject to major stressing whenever in use) being thrown out every 5 years.
On construction/work sites hard hats are supposed to be replaced every 2 years (if you look inside you will find the date of manufacture) due to them ageing in the sun but I am sure very few are replaced.
This article makes interesting reading and helps to answer your question
http://www.roadsafety.mccofnsw.org.au/a/74.htmlThis also makes interesting reading but I think is more about bikecycle helmets than motorbike helmets.
http://www.helmets.org/general.htm