I just started etching, and have chosen cupric chloride (CuCl2 - the resulting chemical of etching copper with muriatic acid + peroxide).
About shelf life:
The great thing about CuCl2 is that you can REGENERATE the solution by adding a little oxygen (fish tank pump, shake bottle, or add a little H2O2), or acid once in a while. Which means you almost never have to dispose of the hazardous waste.
About the previous comment -- the copper in the waste is poisonous even at very small concentrations. "In Australia the EPA regulates maximum copper in sewage discharge to 5 ppm. If you have etchant containing 150g/L of copper, that means a dilution 1:28000 is necessary to meet regulations" -- from http://members.optusnet.com.au/~eseychell/PCB/etching_CuCl/index.html.
That link has tons of information about using CuCl2 to etch copper.
Also this link simplifies the process greatly: http://www.instructables.com/id/Stop-using-Ferric-Chloride-etchant!--A-better-etc/
CuCl2 is slower than muriatic + peroxide, but I think it's worth almost never having to buy more etchant.
Though I haven't tried any very small traces yet. As I understand it slower etching means more undercutting (etching sideways under your resist). As noted -- I'm a beginner.
Also anyone who wants to do this should read about muriatic acid storage -- according to what I've read, even a little gas slowly escaping from the bottle can rust nearby metal.
I think I read that the gas is heavier than air, so if I see any rusting, I'm thinking about putting some kind of base chemical in the bottom of a 5-gallon bucket, and storing the acid in there. Maybe concrete rubble?