Heya, the potential length of hair on any given part of the body, horse or human, is genetically programmed. There are inbuilt seasonal variations in the programme that respond to environmental cues like day length and temperature (hence summer coat/winter coat etc), and genetic programmes tend to deteriorate with age. Anyway, the hair will reach its pre-programmed length, stop growing, and eventually fall out, and be replaced.
Things like conditioning agents may improve the condition, appearance and feel of hair, but not its genetically set potential length. Sometimes though, wear and tear and poor condition can result in hair being worn down and not reaching potential length or thickness, so that's where care and conditioning can help.
Arabian and Andalusian horses are examples of breeds renowned for having long manes. No matter what I do in terms of external care, my current riding horse has a short mane genetic programme and the hair won't grow past halfway down the neck, and that's how it is!
Length and thickness of horse manes can be reduced from their potential through rugging with combo rugs that cover the neck (wear and temperature effects), but it's not been significant enough for me to not use neck rugs on my free rangers in adverse weather.
I read somewhere that there was a mutation in mane and tail hair length in, I think it was an Andalusian, and he was used as a parade horse in some historic European court, with a whole bunch of pages carrying his extremely long mane and tail to keep them from dragging on the ground.