I had outer thighs/hip fat transfer, how much fat will survive?

Dear CThor15,yes, it i true. Only around 60-70% of transferred fat will survive. 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

The survival rate of grafted fat varies dramatically.
By far the most important factor that determines how much of the fat survives is how the fat was grafted and where it was grafted.
Unfortunately the survival rate of fat grafted into areas that were overtreated from previous liposuction tends to not do well.
In order for the grafted fat to survive each grafted a particle of fat needs to be surrounded by live vascular tissue.
Well that's in the area has too much fat grafted into it than the grafted fat is stacked upon itself and does not have access to an active blood supply and the fat will die.
A typical example of this is grafting fat on the outer thigh area and someone who is very thin in comparison to the buttocks.
The buttocks has thick layers of tissue to graft into and tends to have a higher survival rate of fat.
Over grafting leads to a higher percentage of graft failure.
It's difficult to tell in your situation without having pictures for an examination.
If a hip dip was created from improper liposuction then I do not believe it is 70% fat survival rate is likely.
Arias grafted with fat should be protected from pressure for at least three weeks.
After that the fat has either established a blood supply and will remain there permanently while the rest of the fat is dead  and will slowly dissipate.
Best,
Mats Hagstrom,MD