If Regina became Queen when Snow was a child then it could still only be between 10 and 15 years before curse and Regina still looks the same so i would imagine sooner rather then later. She seems to be wearing the same outfit she was wearing in True North. But even if Snow was still a child during the time of Belle and the Beast this could at the longest be 15 years before curse and i think since Regina looks the same it is most likely sooner then that.
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