No, the Montgomery Bus Boycott did not last six months. At least, it did not end after six months. It lasted longer than that. It lasted almost exactly one year.
The boycott in Montgomery is said to have started on December 1, 1955. That was the day on which Rosa Parks was arrested. Of course, the full-blown boycott did not start right away. However, that is the date usually given for the start of the boycott. The boycott did not end until December 20, 1956. In June of 1956, the segregation law that Montgomery had was declared unconstitutional. The city appealed and the boycott continued. In December, the Supreme Court declared the law unconstitutional. On December 20, the city of Montgomery passed an ordinance desegregating the busses and the boycott ended.
Thus, the Montgomery Bus Boycott lasted slightly more than one year.