I did this a long time ago. Lost a lot of weight, then quickly gained it back.
I would advise once you get past the 7 days to then go immediately to calorie counting. I know it is a drag, but you will keep off the weight you've lost plus continue to lose weight. You will be at an advantage going into it this way because you will have already cut back your appetite by doing the cabbage soup thing.
I don't know your weight, height, activity level, etc. You'll need to know all these things in order to calculate how many calories you should be eating daily. For me to lose I need to eat 1500 calories a day. If I exercise, I can eat a little more and still lose. Generally, I've been eating around 1700 calories a day and am losing steadily.
There are a lot of free calorie-counting apps available out there that are very good. I use MyFitnessPal. I'm sure all of them are similar and they really do help.
Best of luck! I'm 3 weeks into my diet so far. 12 more weeks to go!