That is tricky with gold toner just for a very practical reason: it is insanely expensive to use gold as one shot. I use gold a fair amount to cool down lith prints. I tone until it looks right for my application. The good news is that if you see a color change and it stops changing, you've hit some sort of end point. I'd just tone until you don't see additional change and add 10 minutes or so. That should be close to the "true" endpoint. Although, I have to agree with the others...I think a well processed print will last at least a hundred years. My prints don't deserve to live that long...(: