This is a very tough question to answer specifically. It's like asking, "How many gallons of water does it take to wash a car?" It depends on the size of the car, how dirty it is, how you wash it (e.g., hose, sponge, bucket), and how much wasted water missed the car completely.
The biggest factors to consider are surface area, surface condition, wood species, application method, and amount of waste from the process. Also, if you're using a 2-step teak cleaner, you will need to factor in the second component.
Surface area is a pretty basic concept; length multiplied by the width of a surface gives you the surface area. To determine the total surface area, add up the surface areas of all of the sides and surfaces you intend to clean.
Teak that's gray, soiled, and dirty or greasy may require more teak cleaner for adequate cleaning. You may even need to do a second application to get the teak clean.
The amount of teak cleaner will also vary per wood type. For example, these chemicals will generally also work on other oily hardwoods such as mahogany. The more dense the wood, the less teak cleaner can be absorbed into the grain. The condition of the wood presents a similar consideration. Soft, dried, open grain can absorb a surprising amount of teak cleaner compared to a more solid and dense area of wood grain.
Wasted teak cleaner is also one of the biggest factors to consider. The concept of wetting out the wood with the cleaner is pretty basic, but, naturally, some of the teak cleaner will drip away or splash off of the surface. Brushing the teak cleaner on, pouring it onto the wood, and using a sponge or abrasive pad will also absorb some of the teak cleaner.
If you are using a 2-part teak cleaner, you'll need enough of the cleaner and brightener components. The first component (the cleaner) gets dirt and grime out of the grain, usually leaving the wood dark and covered in the grime. The second component (the brightener) is designed to neutralize the first component, so when the wood has been rinsed off after applying the second component, it dries to a bright, golden tone. In addition, having a little extra of the second component can be helpful, as it will remove any staining left over after using the first component.
At the end of the day, it's a good idea to be very conservative about how much teak cleaner you'll need for your project. Most brands also sell teak cleaner in quarts or gallons.
Many people still want more specific yield before starting a project. Typically, a good starting point is 15-25 square feet of wood per quart of cleaner, and 60-100 square feet per gallon. However, it is strongly recommended to be very conservative, depending on application method and condition of the wood. We also recommend budgeting for waste.