Your question regarding how much fat is needed to fill the cheeks and forehead lines is tricky. First, everyone requires a different quantity depending on how deflated they are. Further, every surgeon harvests the fat differently so their yield of fat survival is different. Hence, how much they need to inject is going to vary. More importantly, facial fat transfer is a very difficult procedure to perform and considering it is permanent, you will want to make sure your surgeon is experienced (performing several cases per week) and that they get excellent results consistently (see lots of before and after photos). In our practice, it is rare to only inject the cheeks. This can leave you looking imbalanced, so be aware of this. Typically, a full face fat transfer is a better option in order to re-fill all the different parts of the face. Finally, the lines along the forehead are usually treated with botox not fat transfer.