I guess system spec must matter quite a lot here? Is everyone using fairly equivalent computers?

How much will the structure, size, complexity of a scene significantly impact the time that the DOF takes to process too? (EDIT: cross-posted there with hborre above)

I've literally just upgraded to a 3.4 Ghz quad core i7 imac... haven't tried a DOF render yet. I'm planning to...

But certainly any attempts to do it on my previous Core 2 Duo imac, beyond very basic scenes, I ended up just aborting after a significant number of hours.