So, how long do you like your boss fights? If it's too short then it seems pointless. If it's too long, and you mess up after 30+ minutes in, you chunk your controller/keyboard out the nearest window you can find. Myself, I prefer something that takes anywhere from 10-17 min for a boss and around 20 min for a final boss. Keep in mind that these are for a boss that you know how to defeat and have done it before, you're not fighting it blind.