I do not know what my preference is, for relatively longer or shorter hair on men, but since I do my best to keep my personal preferences out of making decisions about my world's culture, that's not too bad.

Really I just decide based on what I know of the specific people's culture and what would make sense in the interests of convenience.
For example, there are a group of men, rather like outlaws, who are very – * thinks how to say it * – well, simplistic when it comes to accouterments. They own nothing but the clothes they wear and their weapons, and they keep it that way. They don't have very many weapons most of the time, too – though you can pretty much count on a knife at least. And that is what they use to cut their hair, necessarily.
That means that it varies in length dramatically depending on how long ago they cut it, and it is always very – er – scraggly. That is only what some of them do though...some of them skip hair entirely and shave their heads (like the ancient Egyptians did – though I doubt they would also wear wigs like they did * grins *). Some of them let it grow as long as it wants. This is a bother for keeping it out of the way, of course, but I think they just braid it and tie it up with a thong most of the time. And cut it occasionally to keep from having to wash it too, probably. * grins *