Jake had his 2nd at 10 weeks (just before he came home). The breeders told me two weeks, but when I phoned the vet he said that at 10 weeks, waiting a week would be enough.
I think it has to do with the area you're in, but from what my vet said it also sounded as if the age at which they had 2nd vac had a bearing too.
I'd do whatever your vet says, to be on the safe side.