It varies quite a bit for different species and different circumstances. For example, bacteria and viruses reproduce very quickly, so natural selection acts very quickly on them. Thus, we can actually track the evolution of strains of HIV inside of human lifespans in a given patient as they evolve inside their host. However, for longer-lived species like Galapagos tortoises (which can live over a hundred years), evolution may take thousands of years. It’s a continuous process, so it’s hard to pin down a discreet point in time where it has occurred. Also important are drastic events which can change a population quickly. For example, the average size of sperm whales has actually decreased measurably since the time of whaling because of the intense selective pressure humans put on the largest whales. Hope this gives you some helpful insights.
Short answer: it depends on the animal and it depends on the circumstances.
Most evolution takes multiple generations, and the length of time will vary based on the animals birth rates (very quickly for small animals, like mice, much longer for animals with low birth rates, like apes). The quickest evolution we have identified recently is in elephants, many of which are now born without tusks to avoid ivory hunters.
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