Varies from channel to channel.
DTV Broadcasters can use their own discretion when determining how much bandwidth to allocate to any given channel. Overall bandwidths are regulated and can not be exceeded. Cable, satellite operators and terrestrial broadcasters may decide how to apportion those set ranges themselves. HDTV signal also must share the limited room with standard definition broadcasts.
Too many channels squeezed into a limited amount of broadcast spectrum will reduce the picture quality. To remedy the problem broadcasters may allocate more bandwidth to certain channels and less to those channels which do not require much room. For example, a fast paced action movie channel may require lots of room in order render artifact free video. A local information text only channel may require very little bandwidth.