The more complex your models and effects and the longer the movie, the longer it takes to render.
I still remember it used to take a whole night to render a few second's simple movie on a 486, so things have gotten much faster these days. It still will always be a bottleneck in 3D modelling, because so many calculations need to be made.
Try rendering a still image before you render the movie, if you just want to have a look at your model.