
I vetted two of the people well in advance of the mating. They knew the mating was planned - just waiting on my girl coming into season, but we talked well in advance, asked all the questions, kept in touch. When it was taking longer than we expected I let them know of other litters, but was lucky enough that they stuck with us.
I don't think it's ever too early, and the benefit is if it doesn't work out for you, you will feel comfortable enough to pass them on to someone else you know who may have a litter for them (who of course would need to vet them to their standards).
If I get an enquiry, even if I don't have pups, I tend to chat with them and ask questions to get a feel before I refer them on. Sometimes I've talked people out of my breed.