Depends on your cal. intake and how many cal's you need.
An average female would need about 2000cals a day, to lose one pound you need to BURN 3,500CALS more than you ate. So, simple math, to lose one pound in one week you would need to burn 2,000cals a day, and eat 1,500cals. Thus creating a shortage of 500cals each day, for seven days straight=3,500cals burned=one pound loss.
Depending on how tall you are, how active you are, how much you weigh.... you can estimate how many calories you need to maintain your weight... then go from there. A good guideline is to not eat less than 1,200cals a day as your body also needs cal's to work, breathing, your heart, your other organs all need food.
Slimfast shakes don't have that many cals, if you've had two shakes and one snack I'd estimate you're sitting at about 700cals right now, so if you eat a healthy meal for 500 cals and say have another snack of 150 cals you've eaten 1,350 for the day, I would not go below that. And you should be losing weight if you do that for a while. Be sure to take a break from dieting every few weeks though otherwise your metabolism will drop and your body will get used to not getting a lot of cals, you then gain weight when you start eating normal again.
Oh another great way to make slimfast taste better is to blend half a scoop of powder (instead of full scoop) with a banana/yoghurt/milk etc. you can get creative. The powder foams the shake up nicely and adds taste and depending on what you add you get more of a nutritional value+less sugar. It's a great meal replacement when combined like that. I like to blend in bananas, strawberries, cranberries.. there are so many things you could add! Yum. Now I'm hungry. :s