This question has come up for me this year.
My mums W reg Polo has only done 20000 miles, but the belt is 5 years old.
I spoke to my contact at colbornes in woking, he said change it at 60k, so i will probably leave it at least another year.
Its up to the individual owner really, but a cam belt is made to be tough (if its a genuine part), and should be fine after 5 years.