Bitcoins mining takes a lot of time because it is a complex process. And how long the process will be depends only on the power of your computer. Consider two options:
Processor number 1: CPU, AMD Phenom II X4 810
Processor number 2: GPU, AMD(ATI) Radeon 6990
In the first case, the process of Bitcoin mining will take about ~ 3 years. In the second case, the same process will take ~ 203.
But any adequate person, having even a powerful computer will not want to wait three years to obtain Bitcoin. Therefore, there is such a thing as a "mining pool". The idea is to combine the efforts of multiple computers for mining Bitcoin. And in consequence of the award is distributed among the pool participants.