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To date this sculpture, you could look at pictures and other sculptures and compare the style of clothing. Or take a book about medieval clothing and try to determine the period of the clothing this way. Probably a combination of these two approaches would be a good idea.
Edit: The determination of the clothing gives you a starting point for your research, a date after which the sculpture is done.
The headpiece looks to me like a Hennin which would put it roughly in the 15th century. Also the dress seems to fit in this period, compared for example with this painting from Hans Memling, Portrait of a young woman / bride from the second half of the 15th century.
But I am no expert in this area and would be very interested if somebody knew more details.