Martin Ottman replied:
From the Sea Organization publication "Many are called and few are chosen", (c) 1999, page 7:
"FACT: There is one Sea Org member responsible for every 1.2 million people on Earth."
Page 27: FACT: The combined years on staff of all Sea Org members exceeds 50,000 years of dedicated service to mankind."
I think in 1999 the word population did hit the 6 billion mark. So the per that the number of all Sea Org members in 1999 was about 5,000.
If there had been always 5,000 Sea Org member since its beginning and in 1999 they had committed themselves to 50,000 "dedicated service", it would mean that the average Sea Org member had done 10 years in the Sea Org. As the number of Sea Org personnel grew with the years, the average time must be lower than 10 years.
By my own observation, 50 % of all Sea Org personnel stays 2 years or less in the Sea Org. So within two years you have a 50 % personnel turnover. But I might be wrong on that point.