It really depends on the program and the thesis requirements. It can also depend on whether the Masters is an "add-on" to an existing BS at the same school or you go into a new program. In my case (long time ago), I did a Masters at a different school and it was very typically a two year program--basically one year of classes and one year of mostly thesis/directed study. But, as I recall, there were other schools that offered Masters at the time that were more typically one year programs (and did not require a thesis).