Even a quick image search will show you that Worf has worn his hair something like at least five different lengths:
← just kidding
Clearly, Worf enjoyed a good visit to Mot to switch up the 'do.
Since obviously none of these are Michael Dorn's real hair, each different length is a different headpiece, so there must be clearly delineated times when Worf's hair changed.
Worf had relatively short hair in season 1:
It was perhaps a little longer, but not quite shoulder length in seasons 2 and 3 (first two images are clearly season 2, with Pulaski. Third image is from Sins of the Father):
He grew it out to shoulder length by season 4 (Qpid), and kept it through season 5 (cast shot) and the first half of season 6 (A Fistful of Datas):
But by the second half of season 6, he's grown it long into a ponytail (Birthright, Part II). The quote that The Doc cites below indicates the ponytail debuted in Face of the Enemy but I haven't found a good image of it:
Which he then keeps through the remainder of TNG and all of DS9, as far as I can tell. (Liaisons, Let He Who Is Without Sin..., "Insurrection")
This final screenshot is from the alternate future timeline shown in the TNG finale, All Good Things.... It takes place about 20 years later, and makes it clear that, at least in that timeline, he kept it long.
As for Klingon men in general, it seems like long hair has been much more common for non-Augments. (Heart Of Glory, Rightful Heir, First Contact (ENT)) But it also appeared to not be mandatory (Chang in "The Undiscovered Country").
In-universe, it might be safe to theorize that early in his career, Worf was a little less sure of himself and was attempting to assimilate. But as he got promoted and more comfortable on the Enterprise, he grew his hair out in a nod to Klingon tradition. But that's just speculation.
An interesting quote I found on Memory Alpha about this sheds a bit of light on this from an out-of-universe point of view (I've emphasised the important bits):
Gene Roddenberry had initially insisted that Worf have a short hairstyle so as to reflect the "no-nonsense" military look of Starfleet. While Westmore accepted this, it grew somewhat longer over the years. In the sixth season of TNG, due to lobbying from Dorn and hairstylist Joy Zapata, Worf debuted a ponytail (beginning from "Face of the Enemy"). Zapata explained, "[Worf's hair] used to remind me of a Klingon that had gone to the beauty salon; it looked like Donna Reed! Now we tie it back, [Dorn] carries himself differently – he's a whole different person". (Star Trek: The Next Generation Companion 2nd ed., pp. 21, 234-235)
So, more specifically, his ponytail was first 'premiered' (if you will) in 'Face of the Enemy' (S6:E14)
I think he hair has followed a simple profession like any in real-life person may have. When younger as a star fleet cadet and new officer, he likely kept his hair shorter to be within regs or at least an approximation. As he got older and started to explore his Klingon roots and heritage and become more and more involved in Klingon relations and even politics, he started growing out his hair. He kept that usually in a pony tail or loose braid, and the picture of his hair down over his shoulder was simply one time when he was getting in or out of uniform and so his hair wasn't back in a tail. Obviously as he got older his hair greyed and was kept in more Klingon style since he became quite literally a Klingon diplomat in post show eras (flash forward episodes).
