What is the longest you can leave between the dog having its first injection to having its second?

We got a puppy from a breeder and they said he had both his injections and was fine to go out, then a few days later they contacted us to say they made a mistake and he had only had his 1st injection and needed his 2nd. We took him to the vets and they said it was too long since the first one to have the 2nd one and needed to have the 1st one done again before the 2nd one.

However i spoke to the breeder and they said they had another dog at the time that had its first injection with my dog and they had just got the second one done when they realised they had missed it and it wasn't too late for the 2nd injection.

she said the vets are just trying to make more money out of us by telling us he needed to have his first again.

so who is telling the truth? the breeder or the vet?? basically i want to know how long you can leave it between the 1st and the 2nd injections?