I think we need to first focus on WHY a woman's LONG hair would be her glory.
If we look at the context and the culture that was plaguing the Corinthian Christians we notice, and history proves this, that it was in fact a shame for a woman to hair a shaved head.
Having a shaved head meant that a woman either:
1. Was a pagan worshiper, offering her hair to false gods, specifically Aphrodite.
2. Was caught in adultery, and had her head shaved as a result, a common custom of the day resulting in humiliation.
3. Was a prostitute, the adultery would have been grounds for a shaven head, as well as to deal with lice, as prostitutes typically had a much lower hygienic standard.
Therefore, if a woman had long hair, this meant she was neither of these people! She was proper and not immodest. Hence the connection between veil wearing and hair. If a woman did not wear a veil or a covering, she was deemed immodest by society's standards. (Dishonors her head [husband] if not covered)
Sure, we can twist scripture to fit our agenda and look "outside of the Greek" to redefine words, but lets stick to God's agenda and read it for what it says.
So to answer your question, I would say "Long enough not to be associated with the pagans, harlots and adulterers of that day".
Should a woman's hair even today be long enough so she is not confused with prostitutes etc. of the 1st Century?
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Hebrews 13:23 Know ye that our brother Timothy is set at liberty