1 Corinthians 11:14
<Does not nature herself teach us that, while for a man to wear his hair long is degrading to him,
And the next thing it says is:
1 Corinthians 11:15
<a woman's long hair is her glory? Her hair has been given her to serve as a covering.
So Paul, writing under the inspiration of the holy spirit, condemns long hair on males, and his argument is appealing to nature. It seems that the reason men shouldn't have long hair is because it is womanly.
Let's not cop out and say that it depends on culture, because the appeal to nature is irrespective of culture.
Men should not have long hair that is associated with women. I would roughly guess that shoulder length is that limit.
In addition to this we have a cultural view in the west that sees shoulder length hair as less masculine, so I would say men ought to be even shorter than this just for appearances. I wouldn't call it sin though.
Inb4 Samson. He had a specific exception under the nazirite vow. If anyone today wants to electively take a nazirite vow I would say they also have exception.