In general I would say about five to eight years or so. It takes about two years or so for the plant to settle in and become established and then several more years for it to begin to approach full size. Keep in mind that the woody hydrangeas shrubs such as Hydrangea paniculata maintain that branch structure permanently while the bigleaf hydrangeas (H. macrophylla) may suffer severe dieback each winter so they will generally stay toward the smaller end of their size range.
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