Take the number of days it took you to learn your first language (say 20) vs the number of days it took to learn your second (say 15). Your third should be shorter by no more than half the difference (2 1/2 days or 13.) It depends on the language and your actual knowledge of the concepts as some are more complex than others. You will reach a point that it is impossible to increase your speed. I can code in any new language in under an hour, but to become an expert in it is still weeks.