How long does it take for fat to "take"?

Great question!  The first few weeks, the fat cells survive by diffusion of nutrients - this is the crucial period where about 25% of the fat cells do not survive.  Around 1 month is when the new blood supply has formed to the new fat cells and the volume has normalized.Best of luck and hope this information helps!- Dr. Roger Tsai  @DrRogerTsai

Thanks for your question.It takes several weeks for blood vessels to grow into the fat cells that will survive.
Please always consult with a board certified plastic surgeon.
Take care, 
Moises Salama M.D., Board Certified Plastic Surgeon  

There fat take on an area, is different from patient to patient and from the injected area and injected volume. During the fist 8-10 days the fat survives trough the plasma around the injected fat cells, then new blood vessels will develop and start a new blood supply to the injected fat, but that process make take few weeks, 6 to 8 weeks, then you can say the injected fat has its own blood supply.

Dear 12312345123,in most cases 30-40% of transferred fat will be lost. You should expect to see the final results 4-5 weeks after transfer. Fat viability depends a lot of surgeon technique. The fat has to be placed back into the buttocks as soon as possible so that it has a greater chance of survival. It has to be placed carefully so that not too much fat is placed in only one area. Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

Your buttocks and hips will shrink for the first 4 to 6 weeks, and many patients wish more of the fat would stay.