Anyone have any knowledge of the particulars of the license that the broadcasters bought for the spectrum they use. Are they perpetual or for a limited time? J

Anyone have any knowledge of the particulars of the license that the broadcasters bought for the spectrum they use. Are they perpetual or for a limited time? Just wondering if with the networks claim that most of their viewership comes from cable if their use of the spectrum would no longer be considered the best use for serving the public interest. Taking it further, would that allow the government to re-purpose the spectrum for other use.