First, there are two types of music - score and source music. Score is composed (or constructed) specifically for the film; source music is pre-recorded music used in the film. Music is used to enhance emotion, propel action and provide transition.
The obvious answer is "wherever it needs it". The slightly longer answer is the "traditional" silent film had wall-to-wall music. Only the most prestigious movie houses had sound effects people behind the screens, and even back then dead air was not tolerated.
Since you have the option of dozens, even hundreds, of audio tracks for Foley, sound FX and ambiences there is the possibility of having no music at all.