At this point DS, 10, has shaggy but not overly long hair. He had his first hair cut (his choice) around 2.75 years. He went from having long, slightly curly fairly blond hair to having a light brown buzz cut (he got it cut to be in a wedding--- he went to the on-base hair place with his uncle). DD had her 1st hair cut at 3.75 years. She's older, so I'm not surprised that she waited longer.
Since his first cut, DS has had a variety of hair styles. A few times he has grown it out to what I would consider long for a boy (past the shoulders, significantly longer than a shag). He's also been buzzed and had a mowhawk in that time. We're pretty open with our kids about people, so while we allow them to choose their own "look" we do make it clear that other people might say or think something about it. At different times that has played a bigger or smaller part in their decisions.
It's funny you mention that (about your DH having long hair). During DS's time where he most adamanatly wanted long hair (he was 4) he was also very into painting his tonails and wearing kilts. These (hair, toenails, kilts) were all things DP does. It was funny to see a little boy making these choices he saw as really identifiying him with the male gender which most people really don't consider to be "male".