The regulations on hair (both males and more commonly females) are enforced throughout your whole career.
I read on these forums that after phase 2 your are allowed your hair to be longer on top just as long as it is neat and tidy, is this not the case?
3818. Male Personnel
a. Hair. Shall be neatly groomed; taper trimmed at the back, sides and above the
ears to blend with the hairstyle. On the top of the head it shall be no more than 15 cm
in length and sufficiently short at the front and sides that when the hair is groomed and
headdress removed, no hair shall touch the ears or fall below the top of the eyebrows.
It shall be kept above the shirt collar.
The rules are the following:
So, longer than 15cm? No.
Will anyone whip a ruler out and measure your hair? No. Probably not. But if someone thinks it's too long, just go get it cut. Don't start arguing about 15 centimetres...