Usually, I'd guess/assume/overconfidently assert that 90% of revenue happens in the first 5 years. Though I'm open to being corrected.
edit: Based on skimming some numbers at vgchartz.com, it looks like the basic shape of the claim for heavily marketed video games is right, but the revenue half-life is more like three or four weeks.
Those authors deserve a decent price for the film rights to their work. 30 year copyrights would significantly change that.
Although the rights that permitted those films were sold in 1969.
Film rights are a particularly interesting case where they get sold and traded around potentially for decades before something is actually produced, if ever.
Though I'm not sure if that's feasible. Maybe the most use of copyright free books is adaptations?
Absolutely! For artists, the price of their paintings is a function of their popularity, which more often than not comes later in life. When and if it strikes it doesn't matter, on what stage of life the exact painting was created, they all become more expensive.
Is this the common case? It would seem logical that most content makes the most amount of money soon after it comes out. I think for many kinds of content, the first week makes about as much as the rest of the lifetime of sales.
And even in the rare case where some content becomes rediscovered after the 30 years, the creator simply needs to create a sequel, which by having their name attached to it, will be recognized as the official continuation.