Very fast. The canonical example is the peppered moth (Biston betularia) [1] in the UK changing color from white to black and back to white in the last two hundred years. Coal power plants during the industrial revolution covered the surrounding areas in soot and darkened the white lichen covering those trees, which meant that the white moths were more visible to predators. Once clean air regulations and technological improvements eliminated the soot, the gene that turned them black became a liability and the population was once again taken over by the white moths.

The specific mutation event was traced to around 1819 [2] and by the turn of the century, the white variant was all but gone around industrialized cities (someone even wrote Darwin about it in the 1870s). Starting in the 50s and 60s, the distribution began to reverse once the air started to get cleaner and now we're back to a largely white moth population.

[1] https://en.wikipedia.org/wiki/Peppered_moth_evolution

[2] https://www.nature.com/articles/nature17951