In most cases, it is determined by the relationship. Submissives have varying opinions about wearing their collars. Some want to wear it always, while others wear it only when playing BDSM. Submissives decide when to wear their collars based on a variety of factors that come into play, such as whether she feels comfortable wearing it in public, how it goes with her clothing and the type of collar she chooses.
Also, wearing a collar does not always have to be constant. Relationships can change the frequency with which people wear their collars.