The interracial intermarriage rate in Jim Crow America was about 1%. At that rate, if introgression of non-white individuals into whites continued for 50 years, the average person with white ancestry would be 40% non-white. (I suspect this exaggerated figure is due to improper use of a 1% introgression rate per year rather than per generation, but even a few generations of modest introgression does have a notable effect over a very long time period)
In India, caste cemented itself, not just at the Varna level, but at the Jati level about 1500 years ago. This implied an endogamy rate on the order of 99.8% (with 10% introgression over that time period) to 99.9% (with 5% introgression over that time period) to 99.98% (with 1% introgression over that time period).
I think the source is Razib or someone he quoted, but I could be mistaken. I accidentally failed to hit the publish button on this post back on April 17, 2018.