I keep an eye on the UK refurb section and there isn't any definite pattern that I've seen. They obviously don't list everything as it becomes available but I think them adding the latest products is more related to their stock levels of the previous products than any fixed time scales.
The iPod Classics haven't appeared yet but recently they've started dumping the 5.5G at bigger discounts. I'd imagine once they get rid of them they'll start with the Classics. Same with the Macs, once they've got rid of their stock of the previous gen they'll add the latest.