Tread depth is measured in 32nd's. So depending on what kind of tire application, pass or light truck you should have an idea of how much tread the tires has at new. Typically a passenger tire starts at 10 32nd when new. A light truck tire is around 14. Most tires are considered worn out at 3. The dealer or garage you are buying from will give you the percentage of wear remaining. I would not recommend buying a pass tire under 7/32 or a light truck under 9/32. Nor would I recommend buying a used tire from anyone that is not in the business of selling tires. Most reputable dealers thoroughly inspect all of their used tires for damage that could cause a blow out.
I don't know a percentage number, however I do know that if you place a penny in the tread of the tire and can see Abraham Lincoln's head, then it is time to replace your tires. That is always how I have checked my tires when they are close to needing replacement.