There's no cut and dry rule for how much weight you'll lose. I like to think of it more as a reset button for your body - you get back to a more natural state, where you don't crave high fat, rich foods - you crave water, juice, hot tea (hot herbal teas and broths can really help you get past cravings for hot meals).
The first week you may not notice any changes physically, but physiologically you'll be buzzing with sensory information. The first three days are typically the hardest to get through. PLEASE DRINK LOTS AND LOTS OF WATER!!!!! Keep a water bottle with you, and full, and keep keep drinking it.
You'll feel hungry, cranky, lethargic... Then you'll start to feel lighter, and emptier, you'll start to notice your senses are very alive. I wouldn't fast only to lose weight, because some of the coolest stuff that happens has to do with the lightness you feel, the way your senses re-activate, the way you start to feel time stretch out...
My first fast - goal: 30 days reality: 10 days. It's really tough! Good luck, and LISTEN TO YOUR BODY!!!