I'm 17 days post BBL. When can I safely sit, or sleep on my back without damaging the extra fat cells?

It is best to discuss this with your chosen surgeon as each surgeon has their own post operative protocols. I have my patients refrains from sitting or laying on there back for 2 weeks, then gradually easing into it. 

Thank you for the question. I typically do not allow my patients to sleep on their backs for at least 4 weeks and no sitting directly on their buttocks for at least 8 weeks, after a BBL. Each surgeon has their own style in the post operative management of the patient. I would recommend you continue close follow up with your surgeon as they are in the best position to evaluate and treat you. I hope this helps. Best wishes.

I recommend that you refrain from sitting down for more than 30 minutes in the first 1-2 weeks and then you can gradually start sitting for longer periods. 

In general, I allow my patients to sit on their buttocks by about 2-3 weeks after  a brazilian butt lift. You need to ask your surgeon to see what his preference is.