Do you have any idea on your body fat % when you started? If so, you can figure out your approximate TDEE. For instance, according to my last Skulpt scan, I'm approximately 16% body fat at 187 lbs. My TDEE should be roughly 2100 calories. That's essentially what I could eat daily without exercise and not gain fat/weight (i.e. maintenance calories).
If you're only eating 800 - 1000 calories and then doing cardio, you're likely in a steep daily deficit (1200 - 1500 calories depending on body fat % and cardio intensity). So then you'd add your daily deficit up over the 7 days and divide by 3500. That would be the maximum lbs you could lose in fat. The rest would likely be water weight.
I would also say be careful doing too much cardio or too high of an intensity with that large of a caloric deficit. Overdoing intense cardio on a large deficit diet can really wreak havoc with your metabolism and actually prevent weight loss.