What is the maximum alcohol content achievable by fermentation?

Boy, are you in for a disappointment! Bread yeast dies at just a few percent alcohol and leaves an off taste. If you want anything close to 18%, you will have to use champagne yeast or turbo yeast. If you want to distill to 190� proof, your yield is going to be about a tablespoonful. By the time you get enough to get drunk, you will be able to buy your booze legally.