How soon after BBL can I sit fulltime without hurting my graft... like 5 hrs Straight?

Hello dear!!
Thanks for the question and provided information as well.
Each doctor has their own protocol It is very important that you talk to him about your concerns and goals. I recommend my patients not to sit for more than 30 mins. every two hours, for the first 2 weeks, and always with the help of a boppy pillow. If you have any concerns, I suggest you to follow up close with your plastic surgeon.
Hope this is helpful. Good luck :)

I recommend minimal to no sitting for the first 6 weeks after surgery. After six weeks, my patients can sit unrestricted. Please consult with your surgeon. 

I usually have patients wait about 2-3 weeks, but every surgeon is different and you should ask your surgeon directly.