If you constantly have headaches from the weight of your hair, it might be too long.
If your hair gets caught in the tools or machinery that are part of your daily life, it might be too long.
If you hair is breaking off due to it's own weight, it might be too long.
If you keep stepping on the ends when you walk, your hair might be too long.
Otherwise, it's just a matter of your own personal preferences. You might ask your roommmate WHY they think your hair is too long.