rosie - i think he's asking not what the interval is, but how long do they really last safely? there's a big difference there.
skm - to try to give you an idea, when designing this car and making the maintenance schedule, the engineers know the belt will break at some point. its a matter of time. they also know a broken belt will result in major damage. so when to change it?
statistics is your answer. they do a bit of testing and some fun math and figure out when there is a significant occurrence of breaking belts starting. then you dial that mileage number back a bit into a range when virtually none will break. its the point when the belt breaking is a freak occurrence. that's when you replace it.
so your friend with over 300k is one of the lucky few that has not broken the belt by that point. the farther he goes without replacing it, the closer he gets to the day it breaks. the belts can and do last a lot longer than what the OM states. the risk beyond that number would be fairly small, but it would increase enormously the further you go.
as for the 2005 you're looking at, as long as you don't mind replacing the belt it not being replaced at any mileage is fine - as long as it hasn't yet broken or had a few teeth sheared off it. damage has not yet been done.