That's an excellent question, and the answer varies a lot depending on which organism and environment you examine. Remember that species change over time (in other words, they evolve), and individuals within that species do not. This is how species evolve:

Something about their environment changes; variety in the members of the species means that some individuals are better suited to the new conditions than other individuals; the better suited individuals produce more offspring than the other individuals under the new environmental conditions; over multiple generations (if the new environmental conditions persist), more and more of the individuals in the species are of the "better suited" variety, since they now reproduce better than other individuals. The species has thus changed or evolved into one that has whatever traits made them better suited to the new environmental conditions. We consider it a new species if it's different enough from the old one that they tend not to interbreed with each other.

This is why the answer to your question varies depending on what species or environment you're talking about. Species that reproduce very quickly, like flies, bacteria, fungi, even small fish or mammals, have the potential to evolve very quickly, since evolution is changes to a species over multiple generations. Elephants, on the other hand, have the potential to evolve only very slowly, since they might only produce a new generation every 50 years or so. Since environments might change at different rates, that affects how quickly the species in it might evolve, too. A moth species living in a desert might see the same environmental conditions for decades, while the same species living in a temperate zone where there are occasional droughts and floods, warm years and cold years, might evolve because of these changes very quickly.

Here's a cool example: There are different species of ground finches on the Galapagos islands, some with big beaks that eat big seeds and some with little beaks that eat tiny seeds. The species with the little beaks can't crack open the big seeds, and the beaks on the other species are too big to be able to properly handle the little seeds. After only about 5 years of drought, the small beaked birds had evolved larger beaks that were almost as large as the big beaked species! This was because the plants that make big seeds can survive during droughts, and the plants with small seeds cannot, so the small beaked birds had their food source taken away. There was enough variation in their beak sized and they reproduced quickly enough, that after only 5 years or so, they had evolved. Scientists studying them know that they didn't just die out, because when the rains returned, the species changed again to smaller beaks, allowing them to eat the smaller seeds without having to compete for the big seeds with the other species. These scientists were very surprised that this finch species had evolved so quickly, and could evolve back to small beaks just as quickly. They think that the only reason we don't see evolution happening that quickly all around us is because environmental changes don't usually happen that quickly and that drastically.

I hope that answers your question, and I hope it wasn't too long! Keep asking such great questions.

That's a good question, but it's tough to answer. We usually define a species by saying that members of that species can only breed with other members of it. There's a lot of gray area there. Dogs and wolves are different species, but can inter-breed. So we often add that they usually don't under natural circumstances.

The offspring must be fertile (meaning that they can grow up and reproduce). Horses and donkeys can be bred together, but their offspring, the mule, can not reproduce. It can't be bred with other mules, or with horses or donkeys. So horses and donkeys are clearly separate species, but must be fairly closely related or they couldn't be bred together at all.

No species is completely new. Each one evolved from a past species. For example, let's say you have two populations of one species of squirrel. Some are on one side of a river, some are on the other. As millions of years go by and the river carves out a canyon, it gets harder for the squirrels on either side of the river to mix. Each population starts to change due to random mutation or selection for different habitats. Eventually, each population may be so different that if two were in the same place, they would be unable to mate, or would not produce fertile offspring.

Based on genetic evidence, some scientists think that humans began domesticating wolves 100,000 years ago. So, even with artificial selection from humans, after 100,000 years, the two species aren't really that distinct. We're usually talking millions of years for mammals, but if things have short "generation times" (meaning it is not long from their birth until they can reproduce), then we expect things to happen a lot faster. For example, Hawaii's original people brought the banana tree to their islands about 1,000 years ago. There are now several species of moths that live on the banana trees. They are closely related to other species of fruit-eating moths that were already on the island when the banana trees arrived, and there are no other members of their species where the bananas came from. Therefore, it is likely that they evolved in the Hawaiian Islands. In the mid 1800's domestic apples were introduced to the US and a new species of fly (apple maggot) seems to be evolving from the native haw fly.

Plants, on the other hand, can form new species in one generation. This happens when a plant with one pair of each chromosome makes a seed with 4 copies of each chromosome. (Or goes from 4 to 8, and so on.) This new plant is a unique species. It cannot be crossed back to its parents' species. Plants are often self-fertilizing, so it can then start a whole population. Some of our most valuable food crops (such as potatoes) are the result of such a process.

The short answer is that it depends on how the species is forming. It can be a single generation. It can be millions of years. It can be almost anything in-between.

Species that come into existence through genetic mishap and chromosome rearrangement are instantaneous. Species that come into existence through gradual evolution take longer, but how long depends on
(1) the rate at which the two populations to become new species are evolving,
(2) exactly how much contact there is between the two populations, and
(3) how much difference between the two populations is needed before they can no longer reproduce.