Reply #3: Generally it should be done before the first heat cycle, if possible.

[LeftyMom]

The old advice to wait a year or more or to do at least one heat cycle first is outdated, most animals can go in after eight weeks if they're healthy. Your vet may prefer to wait a bit longer (12-16 weeks) but it's not wise to push it much longer than that because allowing sexual maturity to occur increases cancer risk, as well as risk of uterine infections in female dogs.