I often hear students asking me about the duration required for earning a graduate degree such as Masters or PhD. There are degree requirements that specify the number of courses a student must take, in what order etc that might give you an approximation of time. However, let us say a MS degree requires 36 credit hours. As a student if you register for 12 hours a semester, you should be done in 3 semesters. Now, if one of those semesters is a summer semester, you can complete your degree within 12 months.
A doctoral degree typically has more course work, residency requirements etc that will need to be satisfied by every student. Once those requirements are complete, it really depends on how long the student works on his/her research to finish up the dissertation. I have known students work for 7 years and others to have completed their research in less than 2 years.
The duration of degree is not a measure of quality. Students coming from other countries are used to a structure that is set in place with no choices of their own in choosing courses or how many to take in a given semester. It is a very different animal in the US.
For example, undergraduate degrees typically take 4 years in most professional institutions around the world, including the US. However, if you are aggressive student and plan well, there is every possibility to complete your undergraduate degree within 3 years. In fact, there are several schools that offer accelerated programs to achieve this.