In my experience, it takes roughly 25% of the pot volume. I plan on about 1 quart for a 1 gallon pot and 2 gallons of water for a 7 gallon pot.
The reason it runs out the bottom is, water always seeks the path of least resistance. Soil, like a dry sponge, takes time to absorb water. Most of it will run down any small gaps or loose spots in the soil, or down the outer edge. Much of the soil will remain dry.
You must not be putting your pots inside a slightly larger container so they can soak, getting water from the bottom up as well as top down. This isn't required, but if you don't, you will waste water and nutrients. Without a "dunk", it takes a lot longer to saturate the soil since you need to water VERY slowly giving the soil time to expand and absorb as much moisture as possible. Even when I dunk my pots and water from the top, I let the pots soak for a few minutes before draining them.
Most of us have an oddball pot that is a little bigger than what we are growing in. Alternatively, I've used old plastic tubs/buckets that are roughly the same size as my pots. You can use the same size pot if you have an extra. I line the pot with a trash bag so it doesn't leak. Now when you water from the top, any runoff will collect in the bottom and let the soil soak. I often do not dunk 7 gallon pots. They are too heavy to move around in my confined space. In that case I use a saucer to catch the water. It is slower but still lets the soil soak and does not waste nute's.
