Slim fast diet..*help*?

I've started my slim fast diet today. I'm having a shake for breakfast, apple as a snack at around 11, another shake at 2 and then a healthy meal at around 6. However, I work in an office and I'm sat on my backside for 8/10 hours today and am always so tired when I get home! Will it still work if I dont excerise? Also, how long do you think it will take to loose 7 pounds on this diet?