Within one generation, no less. From this 2008 article:
It's a miracle! Blind cavefish, despite having adapted to their lightless environment for more than a million years, can produce sighted offspring in just a single generation, a new study reveals.
The ability was discovered when researchers mated fish from distinct populations that had been isolated in separate caves.
In some cases the first-generation offspring of such unions could see.
Anyway, some thoughts below the cut.
The article treats the ability of cavefish to produce sighted offspring given some particular conditions as exactly that: An ability. Something we should consider cavefish as being capable of. Fair enough, I say.
The first thing I'm wondering is, how should we regard this ability? I think it's a stretch to speak of (techne may jump on my usage of NS here) these cavefish 'being selected for their ability to produce sighted offspring in a single generation' – yet here they are. It seems more natural to chalk the ability up to a consequence of the underlying genetic operations, really: Distinct populations of cavefish differed in "how" they lost their sight, so when they were brought back together it was easier to "fill in the blanks".
The article mentions that "The genetic deficiencies from each parent's lineage were easily overcome by the strengths of the other." What deficiencies? They live in the dark. Cavefish need sight as much as they need bicycles, apparently – or at least this was the case for a long time. Granted, the writer simply knows that we normally don't think of blindness as expendable and is considering cavefish in that context.
But that leads to another question: Are eyes, even sightless eyes, "junk" for cavefish? It's context-dependent. It's not difficult to imagine a change in environment where sight actually becomes useful for cavefish, and not much more difficult to imagine two distinct populations ending up in that same (changed) environment. (The article doesn't mention whether these cavefish will interbreed on their own, or if they have to force them, I'll note.)
I'm not driving home some earth-shattering point here, but I leave readers with this suggestion: Just for fun, consider the ability of these cavefish from a design perspective. If you were assured that these cavefish were designs, how would you regard what's been revealed in this article? What would it say to you about selection, about variation, about species?