At the current limitation of technology, the law of diminishing return will be in set in on non full frame-sized sensors at about 15 MP. And if you ask me, is pretty much today.
But what exactly does full-frame sensors offer besides significantly more expensive glass and body? As far as I am aware of, one advantage - superior images at higher mega-pixel level.
All in all, the APS-C cameras and lenses in the Canon, Nikon, and Sony systems can provide comparable imaging quality at less cost and almost always reduced size and weight. But if you require extremely high-resolution images, if you make really big enlargements, or if you want great leeway in cropping deep into an image, you’ll want a high-megapixel full-frame DSLR.