At What Point After a Bbl Will I Know the Fat Has Attached and Will Not Dissolve?

This is a very good question. There is no scientific answer to  this question. You will need to protect the fat that is transferred to the buttocks by not seating for about two weeks. Initially there will be some swelling   your buttock will  decrease in size by about 10% to 20%.

 

Whatever is the size of your buttocks after three months, will be the size  that you will have forever.

Hello dear!!
Thanks for the question and provided information as well. 
After a BBL, only about 70% of the transferred fat cells will survive, so it is normal to see a minor reduction in your buttocks.At the stage you are in, you are still early on the healing process, it can take up to six months to see your final results, so please be patient.
If you have any concerns, please follow up close with your plastic surgeon.
Good luck :)

After 3-6 months, I think there is little chance for the fat cells to die.  Your buttocks will definitely shrink somewhat until at about 3 months.  Some of this is due to reabsorption of fat cells which did not survive and some is due to your swelling going away.  There is no way to tell which it is.  The most important thing is to do everything right during the first few weeks in order to keep as many of the fat cells alive as possible. 

I have done about one thousand BBL patients and have rarely seen any changes after 3 months and these were always minor. 
Good Luck
Dr M