Homebrew PCBs

> 
> Wondering that myself.  I suppose the easy answer is "when it stops 
> etching to your expectatins".  Also is probably a reason bubbling air is 
> better than H2O2, but bubbling creates corrosive aerosols.
> 



They both work fine. Once it's turned into cupric chloride, it doesn't matter of the peroxide breaks down. You just add a capfull prior to etching if you don't want to wait around for air to regenerate it. It's cool, it almost instantly changes from very dark brownish-green to light green.