I'm not going to argue too strenuously for any particular number, so long as your proposed number is lower than "70 years ranging upward to infinity" - don't look a fantasy gift horse in the mouth, right? - but I'll just point out that even if "A Game of Thrones" were magically out of copyright today, Martin would still be benefiting from the surge of interest. The interest would sell the sequels that were still under copyright. It would sell the sequels that have yet to be written.
It isn't even obvious that Martin himself wouldn't benefit from the additional publicity of giving his earliest books away for free, then selling the sequels. Though I doubt it in this particular case, because of the whole HBO-series angle: presumably Martin is collecting royalties on the TV show, and collected option payments even before the show was made, and if the original book was out of copyright HBO needn't have paid these fees. But the works that are in peril aren't the ones that get optioned for movies and TV: Adapted works stay in print, if only as a form of tie-in merchandise for the show.
If A Game of Thrones were out of copyright, he wouldn't be collecting royalties on the adaptations -- unless he made such an agreement while it was still under copyright.
One side effect of making copyright terms short, as is being discussed, is that studios could freely make movies (video games, etc.) based on all but the most recent works without seeking approval or paying royalties.
And if HBO's adaptation of A Game of Thrones was popular but was relatively inaccessible since most people don't have an HBO subscription, any random person could make their own adaptation of the book and sell it cheaply and widely.
I'm not convinced I understand the full implications of this enough to make a definitive statement about whether it is a good thing or a bad thing, but it's definitely something that should be considered carefully.
It isn't even obvious that Martin himself wouldn't benefit from the additional publicity of giving his earliest books away for free, then selling the sequels. Though I doubt it in this particular case, because of the whole HBO-series angle: presumably Martin is collecting royalties on the TV show, and collected option payments even before the show was made, and if the original book was out of copyright HBO needn't have paid these fees. But the works that are in peril aren't the ones that get optioned for movies and TV: Adapted works stay in print, if only as a form of tie-in merchandise for the show.