For every feature change, you increase the number of renders required by that number...
So let's say 5 skin colours, 5 hair colours and 3 types of facial hair (+1 for none) would make it 5x5x4=100 times the renders. Depending on the GFX card and complexity, that is 600-3000 minutes PER SCENE (10-50 hours!!!!)
(And that is ignoring the file size!!!)