In studies that were done maybe ten years or so ago, they were trying out these natural wetlands to see how they would reduce toxicity. Within months there was a significant reduction. So they estimate--project--maybe a year or two, but that's as long as you don't have any fresh input of new tailings, which would keep the toxicity up.
But one thing that will not disappear is the salinity. Unless you treat the water, it will stay there.