In every age where people used pikes made out of wood, the max has always been around ~6m.

Oda Nobunaga's Nagae Yari troops who employed the longest pikes in Medieval Japan maxed at a little under 6m. Pikemen in the west maxed at about 6m. The Polish Winged Hussars used lances in the ~5m range only because it was constructed hollow.

So by trial and error, ~6m is the upper bound of an effective pike.