Homebrew PCBs

Hi guys I'm new using the homebrew mix of HCL 30% and H2O2 3%, and 
would like to know what is the best temperature for good etch time??? , 
right now when i do a fresh mix, the etch time is really fast around 5 
minutes , but if a week later i want to reuse that mix now clear green 
color my etch time goes up to more than 30 minutes!!!!!! 

Thanks

Bruno