How long your 100% charge on your MacBook will last really depends on your usage patterns and the Mac model you are using. Screen brightness is also one of the most important factors that will affect the duration of one charge — at max brightness, you will see much less autonomy of your devices.
You can find expected battery duration for your model on apple.com if your model is still being sold. For example, MacBook Pro 13-inch currently is listed as expecting up to 20 hours of movie time using Apple TV and up to 17 hours of wireless web.