How long does fumigation take? Does it leave behind a toxic residue? Fumigation typically takes two-and-a-half to three days—if we start on a Monday, you could be back in the house on Wednesday evening. The fumigant is an odorless, colorless gas, not a mist or spray. There’s actually a warning agent that’s added to it so you know when you’re exposed, much like the gas company adds an odor to natural gas so you can smell a leak. The fumigant doesn’t leave behind any residue or odors.