When using Ferric Chloride when should I change the Etching solution ?

Ferric Chloride Etches 1oz (35um) of copper in approximately 60-70 seconds in optimum conditions.  With increasing saturation the etching time increases to over 3 minutes, under etching increases slightly.  Theoretically Ferric Chloride will take up to 65g of Copper per litre, but this limit will not be reached as the Etching time will take to long.  The Etching solution when spent should be neutralised and disposed of according to local in country regulations.  We recomend changing the solution when the Etching time has doubled.