I have read extensively about hair for men in Islam.
Imam Ahmad may Allah have mercy on him, clearly stated long hair is a sunnah for which you'll be rewarded. He was honest enough not to declare it "just a custom" (as one influential Saudi scholar claimed, without evidence of course). Instead imam ahmad confessed that he finds that sunnah to difficult for himself and he rather practices other sunnahs.
I have never read in any hadith about a limit for the length of hair.
The length has rather to do with practical things such as when the length of the hair becomes annoying for a person (depending on the individual) or the hair becomes damaged.
Usually after a long time of growing, the lower part of the hair becomes damaged and it is often necessary to cut of this lower part so that the hair can grow further healthily.
Importantly the descriptions of the hair if the prophet, sallalahu alayhi wasalam, and the sahaba shows that they did not have one 'standard' length.
Usually it is described as long (e. g. prophet, salallahu alayhi wasalam;; abu ubaydah among others) or totally shaven (like 'ali, radhiallahu anhu).
We also hear that they used to braid their hair, which can only mean that this hair must have been very long, much longer than shoulder length.
I know from experience and other people that it is practically impossible to properly braid shoulderlength hair. It looks ridiculous and feels anoying.
One description of abu ubayda, radiallahu anhu, where his braids are said to be 'flying around' while riding an arabian horse clearly indicates that the hair must have been veeeery long, (people with braids will understand) and abu ubaydah was promised paradise. so it cannot be womanish.
The hair descriptions of the muslims are typically compatible with the statement of the prophet salallahu alayhi wasalam, in a hadith about a boy with partially shaven head:
"Shave all of it or leave all of it [to grow]."
leave it [the hair] means exactly that. dont mess with it by cutting here and there like the non muslims... it will grow by itself.
The prophet salallahu alayhi wasalam, did not tell us to keep the hair at a specific length. he was described by other people on some occasions. once he had shoulder length, once to the ears, then braided (indicating very long, significantly longer than shoulder).
that doesnt mean he "kept" it at those length or ordered us to do this, instead it is a physical description of him by other people at a certain point in time.
His hair was simply growing and growing, this should be be the reason why he is described with different lengths.
It's not like you can decide that your hair grows ten centimeters over night as some naive people indicate.
I have not found any evidence that the muslims kept (mutilated) their hair like the men nowadays do, very short.
And this does not surprise me at all because this reprehensible practice was invented by the Western powers around 100 years ago (sometime around WWI).
look at very old photos of muslims. You will find many men with long hair, often braided, very long even, like normal people did.
Unfortunately very soon after the non muslims made super short hair popular the muslims followed.
Have you never wondered why 'muslims' nowadays look exactly like the non muslims? It is basically impossible to tell them apart.
In addition the short hair has led to many styles that are clearly haram.
What to do? Follow the guidance of the prophet, salallahu alayhi wa salam, either shave your head completely or let it grow long, whatever length suites you, also you can braid it.
Both shaving or long hair look much better than short hair, people are just not used to it and they rather follow the sunnah of the colonialists.
By the way, the claim that long hair and braids was just a custom that was typical among arabs is wrong.
The many ahadith about hair seem to indicate that different social/religious groups among the arabs had their own hairstyle and within a group not everyone had the same. So there was massive variation in styles and length among arabs!
And it is interesting to note that muslims are typically described as long haired.
This is what I personally remember from scholars I read about as well as the hadith and I hope what I say is correct, if there are mistake I hope that you can forgive me.
May Allah have mercy on us.