approximately 10,542,387 litres
6,859,000,000 total population (Nov 2008)
Women live longer than men so men only occupy 49% of the population, which means...
3,361,000,000 males in the world
20.45% of those males are younger than 14...
2,674,000,000 males in the world are at least 14 yrs old.
If we assume that each of them is ejaculating an average of 0.8 times per day (Age 17-28 average more than once per day whereas older men ejaculate less, not to mention 5% of men say they don't masturbate), then each male is ejaculating about an average of .003943 litres per day, which means...
10,542,387 litres of human semen are ejaculated per day.