Depends on the breed / size of the dog. Large dogs have shorter lives. I like large dogs. About five years is a good age to get a puppy in that context. I wouldn't wait more than seven years.
Personally, I would say 3-4 is a good gap. That is what we have done with our last 3 pups, I wouldn't want them to be much farther apart as I feel like they wouldn't play or bond quite as well when the older one is more set in their ways and has less energy for puppy shenanigans