Personally, I think the hysteria over 2 year belt changes is just that, hysteria. I've left mine for 4 years before (though low mileage) and never seen any sign of impending doom and could probably have got another couple of years but 'bottled' it.
I think there were a few documented failures of 4V belts in the early days that prompted all this and that the belts in recent years are much improved.
However, to avoid any chance of catastrophic failure I wouldn't advise anyone to leave them too long, especially if you're not scrupulously checking them regularly.
As a bit of light entertainment, here's a picture of a cambelt that I recently had the pleasure of changing (along with all the valves) on my Son's car, it failed at around the recommended mileage for replacement though probably a couple of years past the time, note it didn't actually break and was still tight around the cams, it just lost it's teeth around the crank pulley.
