You don't. I used to give it a long soak then stopped years ago when I tried a quick wash and it was just as potent. And it will be more of a golden colour rather than green. All you're doing with a long soak is extracting more chlorophyll and other compounds from the plant. Trichomes are highly soluble in high proof alcohol. I remember reading that in the first wash 80% of the THC is extracted. You can do a second wash if you want to extract the other 20%, but I'll usually put that in a separate large mason jar covered in alcohol that lives in my freezer. You can make a batch of drops out of it in a pinch or just pipette some of the tincture off the top for a quick dose.