Just long enough to put into a low ponytail. No bangs - I hate it when my hair gets into my eyes.
Nobody has tried to pull my hair (except jokingly, or demonstrating "what could happen") at the dojo. It's been knelt on, but that doesn't happen often.
By the "keep it short so people can't grab it in techniques" theory, we should probably all have our ears lopped off, too. If you like wearing your hair long, learning to deal with that possiblity that it could be grabbed by an attacker would make more sense that cutting it.
Of course, wearing it really short has some practical advanges: easier to wash/dry, doesn't get knelt on, etc. If you want to wear it short, go for it.
My mid-neck-length has some non-Aikido practical advantages: still too short to tangle, long enough to keep sun off my neck, and covers my ears, which is great for working around bugs (mosquitos!) in the barn/yard.
