Well, you can get about a quarter to a half a cup of rendered fat from your typical grocery store broiler/fryer or roaster, respectively, depending upon weight, of course. I'd say half a cup of fat would yield at least that much soap, since you are just adding to the volume of the fat with the lye solution and any added ingredients such as herbs, oils, etc.
I know that any fat can be saponified, but would a fat that is normally liquid at room temperature turn into a solid soap, or would it be a liquid soap similar to a liquid hand soap? Or, can you add something to it to make it solidify?