"Months to years, it depends". Many of the famous Renaissance sculptures took between one and three years. Almost three years for Michelangelo's David (september 13th 1501-early spring 1504), about two for Moses, and almost two for the Pietá (1498). Donatello spent more than two years on each of Zuccone, Saint George and Saint Mark, working with several assistants.

Sources: http://sourcebooks.fordham.edu/basis/vasari/vasari26.htm "Donatello: Sculptor" by John Wyndham Pope-Hennessy