Homebrew PCBs

Been using the same 17 fl ounces of etchant for 5 boards, roungly 45 
sq inches.  1/2 10z copper the other 2 oz copper.  Thought the last 
two took a little long (both 2 oz copper).  Got me thinking how much 
should 17 fl ounces be able to etch? 

Thanks,
 Phil