We shouldn't confuse causation with correlation, but the slowing of "big" ideas is roughly correlated with the outer limit of the patent age. 1900 seems archaic, but 1960 still seems relevant (Minus those pesky, non-consumption-friendly social movements). Cars, telecommunications, miniskirts. It's like we're seeing our culture enthrone its mores in the way Americans know best: litigation, and the legal system. Our culture is slowing down in some sense, but in reality it's just becoming "legitimate" and stable.

For example: where's the big new music scene? It's not because there's no musical avenues left to investigate. It's because our culture is so laden with ideas, we're incapable of propagating new ones into the mainstream. Why not an exploration into pure acoustic tonality, a la Yoshida Brothers? Because our culture is not capable of accepting it. We've become static. Hell, the whole concept of a "hipster" was in thrifting old, expired cultures. The vogue was in retro. Bruce Sterling, eat your heart out.

So, to answer your question: if we don't see any major cultural revolutions? Soon, but genesis will always be around 1900-1960.

If we get cyborgs and AI? Never. It would be like trying to rehash ragtime. Eventually patents will melt away, and reform at the beginning of the new age.

Basically: patents are the cultural realization that old ideas are still valid and commercially viable today. I don't like that, but there it is.