It very much depends on your initial fatness. Unless you're flirting with < 12% BF right now, it's certainly more than the rote "1-2 pounds per week." Let's assume you're an average American male fatass of average height, I'll pull a number out of my ass and say you might lose 11.34876 pounds that first week.
For maximum change on the scale, it's tough to beat a ketogenic diet. By burning through your glycogen, you'll dump 5-7 pounds of water right there. Can't really recommend an outright fast for very long, it's not going to go well for you unless you're hugely fat.
Check out the PSMF. Lyle McDonald wrote a fairly thorough e-book on the subject. It at least attempts to hang on to some of your muscle tissue.