I can see some confusion surrounding the custom of Spartan hair when scrolling through this thread, so I thought I'd try and provide some closure. Here I provide an excerpt from 'The Histories' authored by Herodotus in which he claims to know roughly when the Spartan warriors started to maintain long hair.
As the city of Sardis lays besieged by King Cyrus of Persia, King Croesus had sent messengers to the Spartans for assistance in the matter...
"It happened that just at this time the Spartans were engaged in a quarrel with Argos over Thyreae, a place in Argive territory which the Spartans had cut off and occupied. (The country to the west as far as Malea once belonged to the Argives, including Cythera and the other islands in that neighbourhood.) The Argives marched to recover their stolen property, and agreed in conference with the Spartans that three hundred picked men a side should fight it out, and that Thyreae should belong to the victors; the rest of the two armies were to go home without staying to watch the fight, lest either side, seeing its champions getting the worst of it, might be tempted to intervene. On these terms they parted, leaving behind the men chosen to represent them, and the battle began. So closely was it contested that of the six hundred men only three were left alive - two Argives, Alcenor and Chromios, and one Spartan, Othryades - and even these would have been killed had not darkness put an end to the fighting. The two Argives claimed the victory and hurried back to Argos; but the Spartan Othryades remained under arms and, having stripped the bodies of the Argive dead, carried their equipment to his own camp.
Both parties met again on the following day, when they had heard the result of the battle. For a while both Argives and Spartans maintained they had won, the former because they had the greater number of survivors, the latter because the two Argives had run away, whereas their own man had remained on the battlefield and stripped the bodies of the dead. The argument ended in blows, and a fresh battle began, in which after the severe losses on both sides the Spartans were victorious. From that day the Argives, who were previously compelled by custom to wear their hair long, began to cut it short, and made it an offence against religion for any man to grow his hair, and for any women to wear gold, until Thyreae was recovered. The Spartans also adopted a new custom, but in precisely the opposite sense: they used not to grow their hair long, but from that time they began to do so. It is said that Othryades, the sole survivor of the three hundred, was ashamed to return to Sparta after the death of his companions, and killed himself at Thyreae." - (1.80).
The siege of Sardis is dated to 547 BCE, and so it wouldn't be unwise to assume that it was around this time the Spartans adopted this custom.
