I didn't gain any weight on SSRI's. You simply have to adjust your eating habits to your decreased metabolism. Anyway you can lose 1 to 2 pounds a week or more depending on your metabolism current weight etc. I've read that generally 1 to 2 pounds a week is a healthy way to lose weight.
I'm not so sure your body will dip into its fat stores at all unless you change your exercise levels and/or diet. After all, if you're still providing medium/high caloric intake and not burning much more than you take in, there's no reason for your body not to keep the fat for that "rainy day".