Some stop reading at a certain number, others read through the entire essay and then penalise you for its length, others read through the entire essay and then decide whether the argument is worthy enough for the length to be ignored, and others just don't care provided that it's within reason (their idea of within reason, of course). I'm sure that there are other ways that an academic/marker may deal with an extended response, but they are the four that I have encountered. Given that at this stage you do not know what will happen, you would be best to ask the person who will be marking it or the person setting the marking guidelines (if possible). It's safer than hoping for the best.