Bottom line is you want to have enough to know what you're getting yourself into, be able to establish that medicine is a good fit for you, and essentially show this to adcoms by proving you have had some exposure to different clinical settings. So really it's subjective in that sense, you definitely want to have some baseline hours, and more hours will not hurt you, but they also may not help that much either. The important thing is what you take away from the experience and what you can translate into your PS and speaking about medicine passionately in interviews and perhaps even having some stories to tell if asked for specific examples and things of that nature.
I would agree about 50 hours of shadowing is sufficient, perhaps in a few different specialties if possible to broaden your perspective, as well as some hospital and/or clinic volunteering of about 100 hours (100/0 or 50/50, however you want to split it up). Beyond that I'd say it's up to you. Remember it's less about the hours and more about the quality of the experiences.