That depends on the length of the source, the compression/quality of the source(a BR source will take a lot longer than a DVD), the app that you use, the level of compression that you choose for the output and exactly which 2012 MBP you're referring to.
there are thousands of settings that can affect the render times. depends on the quality of movie you want. i suggest going to a video encoding forum for answers. some people actually make a lot of money simply doing what you are asking about.