Never or Almost Instantly, Depending on how Fashionable the Rabbits are.
Humans have a hellaciously hard time telling if whole species are sapient. Look at whales, dolphins, some (all?) of the great apes, certain parrots.... We just can't decide. About whole species. Who are collectively doing sapient things. A single warren of sapient rabbits is one housing development or parking lot away from extinction. Wild rabbit warrens follow a boom-and-bust population cycle anyway, so even a successful sapient rabbit warren would (unless they get together to start murdering every predator around AND learn some hygiene to stave of disease) just attract more and more predators.
Meanwhile as this goes on the local humans might think the local rabbits are "smarter" than normal, but certainly not sapient. They don't talk! They don't build cities! (at least nothing humans can see, warrens being underground. And even then they're unlikely to be so much better humans would notice.) They don't paint or do anything "civilized"! They don't talk! They're just animals. Smart animals, but not sapient. Remember a suddenly-sapientwarren (barring hand-waving) isn't going to also have complex societal things like modern governments, research divisions, militaries, embassies, or whatever. They're going to be the rabbit equivalent of humans at 100,000bc. Except your bunnies can't hold spears because they have thumbs and don't need clothes! So it would take only the most extraordinary luck and a huge amount of time for humans to realize the rabbits are sapient.
Your rabbits, for cultural reasons outside "to ID ourselves to humans" (after all, we can't spot sapient species so why expect rabbits to ID us?) use dyes or personal ornamentation. To my knowledge none of the other possibly-sapient species on the planet do this. Decorate nests yes, decorate themselves, no. If rabbits were suddenly seen/caught wearing intricately nibbled wooden paw-rings with regular or animal designs, or with woad running in patters all over their fur, you'd get the media and scientists interested pretty quickly. From there it'd be easy to realize these rabbits are somehow chimp-smart. (some sort of Rabbit expert or studies should get there fast.) Then sapience is a shorter leap to make, because you can obviously see the warren is miles more intelligent than the rest of the species.