A thread for anything about Eldarin hair! I'll start with male Elves noted specifically as having long hair:

 Celegorm (noted in Quenta Silmarillion, mid to late 1930s version, IIRC). Olwe (Quenta Silmarillion early 1950s). Celeborn (draft text [long white hair]. Published text [long silver hair] 1954, The Lord of the Rings). Fingon (noted 1968 or later, The Shibboleth of Feanor).

Anymore? please add. For myself I picture a lot of male Eldar with long hair (though what's 'long' can vary from one person to the next of course), but again these are just the examples that have quite specific descriptions to point to.

Glorfindel's (whose name means 'Golden-hair' as most people know) hair is described as he rides his horse (Fellowship of the Ring) -- which description might indicate long hair in my opinion. In the unrevised very early Fall of Gondolin a Balrog pulls him to his death by grabbing his hair (not that that proves he had long hair necessarily).

In a very early picture (1928) Tolkien drew Beleg and Gwindor (Flinding) with what I would call 'long' hair, but he also looks to have given Beleg a beard at this time (or arguably so), and I'm not sure the beard detail would have been retained according to later comments about Elves and beards.