Using PBW to clean a brew kettle

I misread a recipe and realized I have to use my bigger kettle that hasn't been used in years. There's some brown in it from previous uses. Not caked on malt, just discoloring. My yeast starter is going and today is the only day I have to brew so I can't soak overnight. How long should be good and at what concentration (per gallon)? I know any microorganisms will be killed in the boil but I'd like the pot to be clean

Just throw an oz or two in a gallon or so of water in the pot, let it soak for about 30 min and give a quick scrub out. Should be fine.