It's not a fixed time. It's based on the country dropping below 50,000 immigrants in a 5-year period.
For example, Colombia became ineligible in 1996 and hasn't been eligible since.
Poland became ineligible in 1998, the became eligible again in 2002, ineligible in 2007, ineligible in 2013.
And several countries like China, India, Canada, Mexico, and the UK (except Northern Ireland) have never been eligible.
It's not about race, culture, or location. It's simply if there are more than 50,000 immigrants from a country in a 5-year period, they are not eligible as they are already providing a large number.