Answer the question from the given passage. Your answer should be directly extracted from the passage, and it should be a single entity, name, or number, not a sentence.

Passage: Some theories developed in the 1970s established possible avenues through which inequality may have a positive effect on economic development. According to a 1955 review, savings by the wealthy, if these increase with inequality, were thought to offset reduced consumer demand. A 2013 report on Nigeria suggests that growth has risen with increased income inequality. Some theories popular from the 1950s to 2011 incorrectly stated that inequality had a positive effect on economic development. Analyses based on comparing yearly equality figures to yearly growth rates were misleading because it takes several years for effects to manifest as changes to economic growth. IMF economists found a strong association between lower levels of inequality in developing countries and sustained periods of economic growth. Developing countries with high inequality have 'succeeded in initiating growth at high rates for a few years' but 'longer growth spells are robustly associated with more equality in the income distribution.' Question: How long does it take for the effects to manifest as changes to economic growth?
several years