The only way to know how "strong" your CuCl etchant is, is to use a
hydrometer. The copper makes the solution more dense, by measuring
the density you can determine the copper concentration.
See
http://members.optusnet.com.au/~eseychell/PCB/etching_CuCl/for all the details.
Note: my solution looks like the photo at the top of Adam's page.
"Spent" etchant is opaque BROWN. This may look like dark green at first
because it's mixed, but see Figure 5 on Adam's page.