Looks like current technology is 1000x1000 per eye at 90Hz: http://www.digitaltrends.com/virtual-reality/oculus-rift-vs-...

8k x 8k per eye at 120Hz is 64x more pixels at 1/3rd increase in frequency ~= 85x more processing power. Making the (maybe faulty) assumption of doubling processing power every 2 years and that current setup is processor limited, this sort of processing power is ~13 years away.

Same computation but with 4k x 4k per eye predicts ~9 years of progress needed.