The S3 hit the 20 million milestone in a hundred days and it seems that the S4 is selling about 1.7 times faster than the previous generation. According to the Korea Times, Samsung will shortly make a formal announcement, but for the time being the numbers are still unofficial.
It should be noted that the S4 rolled out in May, although many markets did not get it until June. In other words, it took Samsung less than two months to hit the 20 million mark, and for much of that time it did not even tap some markets.