What is the shortest time period over which evolution has been observed? Is evolution something that always takes many many years or has it been observed over short time periods in fast-breeding animals?

Evolution of complex animals like horses or felines takes millions upon millions of years. You canusually only track it with fossils. Bacteria? Sometimes less than two weeks. Maybe shorter. Shortest record; around 24 hours.

Bacteria multiply fast. Like as long as their is medium and space to sustain them, they just keep going. You can observe evolution occurring in bacteria because you can see so many generations arise and adapt in a very short amount of time. And there is a lot of them, so there is a lot of chances for mutations to occur, and for population recovery in case of an incident, and a large amount of specimens for natural selection to enact on.

The best example is resistance to antibiotics, a well documented phenomenon and massive problem. Say you dose a group of e coli with a drug. The medication kills e-coli. Well, most of them. A couple have a mutation that make them resistant. So those couple of ecoli, in a a very small amount of time can produce hundreds of generations with their DNA. You then have all the generations of the mutated bacteria, naturally selected against the meds. And bacteria can swap plasmids (sort of like genetic plug-ins) so they can pass that resistance in some cases to others that way.

TL:DR, bacteria can evolve very quickly and rapidly because of their almost exponential reproduction, and the method of horizontal gene transfer allowing natural selection on a large scale in an observable amount of time.

The shortest time period over which evolution has been directly observed is typically a few generations, which can range from a few months to a few years. Evolutionary changes have been observed in fast-breeding animals such as bacteria, insects, and certain plants within relatively short time frames.