Lots of things in life we can't predict. I think if there's no hearing loss and only hissing, you'll be fine. But no one can tell you how long you have to wait.
Agree with Vassili - please cherish the fact that your hearing is fine! :) Hissing is likely to become quieter or go away completely, provided you don't experience any other acoustic traumas.
Take solace in knowing many people have done alot worse than this and walked away fine. Just bad luck and not your fault per say. Just as you have prob gotten away with things unbeknownst to you, that others have woken up too and thought WTF.
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