What is a good goal for 6 months to loose weight?

I am a female weigh 213 pounds and am 5'6". I am planning a vactaion to California in 6 months and have decided it isn't much fun if you are fat and can't enjoy yourself when your around all skinny people. Anyways, I have a bunch of weight watchers books and I am going to do the points system. What would a good goal weight be? I was thinking 170 in 6 months, that would be 40 pounds does that sound too hard? I have a very buisy life. I am going to buy an exercise bike and I have a weight machine and a sit up thing that works real well. I just want to do it right this time and keep it off. I have lost weight before and I eventually gain it back.