There is no doubt that evolution takes a very long time to become visible. Generation after generation can come and go before any changes in a species can be seen. Since it is usually longer than the human life span, it is not a certainty exactly how fast evolution occurs. The two generally accepted ideas for rates of evolution are called gradualism and punctuated equilibrium.
Based on Geology and the findings of James Hutton and Charles Lyell, gradualism is the idea that large changes are actually the culmination very small changes that build up over time. This is seen often in geologic processes and when Charles Darwin first began formulating his Theory of Evolution, he adopted this idea for how evolution happens over very long time periods.
The fossil record is a piece of evidence that supports this view. There are many transitional fossils that show structural adaptations of species as they transform into new species. The geologic time scale helps show how the species have changed over the different eras since life began on Earth.
The other generally accepted hypothesis for the rate of evolution is called punctuated equilibrium. Punctuated equilibrium is based on the idea that we cannot see changes in a species, so there must be very long periods of no changes of species. That is the equilibrium part of punctuated equilibrium. However, we do know that species do change, so there has to be a period of time where those changes occur. Punctuated equilibrium asserts these changes over a relatively short amount of time "punctuating" the long periods of equilibrium.
Strangely enough, the fossil record is also cited as evidence for punctuated equilibrium even though it is also given as evidence for gradualism. Proponents of punctuated equilibrium point out that there are many what are considered "missing links" in the fossil record. If punctuated equilibrium is the correct model for the rate of evolution, those links never really existed to begin with, so that removes the issue of missing links in evolution.
At this time, neither hypothesis is considered more correct than the other. More evidence will be needed before either gradualism or punctuated equilibrium will be declared the actual mechanism for the rate of evolution.