Assimilation depends only on goverment. In Communism and Democracies every foreigner has a chance of 4% to convert to your nationalty, in Feudalismus the chance is 3%, in Anarchy it is 1% and every other goverment has a chance of 2%. So Com and Dem need averagely 34 turns to convert 75% into your nationatly, Feud needs 46 turns and the rest but anarchy need 69 turns on average to convert 75% into your nationality.
Doesn't the ratio of "foreigners to natives" have an influence on assimilation as well?! I thought I read somewhere that foreigners can be assimilated only if that ratio is < 1. (Which means at least 51% if the citizens need to be of your nationality, before assimilation can begin.)
But not sure, whether that's true. And I can't remember where I read it...