Homebrew PCBs

HCl alone is transparent, but HCl alone will *not* etch copper.  It
needs an oxidizer.  Any oxidizer will do, but H2O2 (peroxide) is dirt
cheap and readily available, so it's the oxidizer of choice.  In a CuCl
etch (the opaque green stuff), the CuCl itself (when green) is the
oxidizer.

The life span of an etchant depends on two things: (1) how much of the
"donor" material is used up, and (2) how fast volatiles evaporate or
decay.  In an HCl etchant, the chlorine is both used up by the etch and
can evaporate, and the H2O2 breaks down over time.  A CuCl etchant lasts
longer but you still need to add O2 (air) and HCl as they're depleted.
Any FeCl etchant will be "used up" over time, but in general won't "go
bad" due to evaporation or breakdown.  I have some FeCl etchant that's a
few decades old that still works.