Some prepare multiple demo reels of different lengths; a shorter reel can work as a hook, with the longer reel kept as a secondary resource should a potential employer or client ask for more samples of an animator's work. At times you may have to custom-tailor your reel to the specific length requirements of a situation.
If your demo reel is two seconds over or four seconds under an exact length, it's really nothing to worry about. What matters most is creating a short, punchy sample that demonstrates your best work and showcases your talents effectively. Creating a demo reel can be a good exercise in self-editing, but can also help you identify your strengths and weaknesses as you evaluate what does or does not present your skills in the best light.
