Consider two things: Positioning and Type of anesthesia.
1. As Dr. Canales stated, positioning is key. For example, breast procedures and fat grafting to the buttocks. Because preserving the fat grafts requires avoiding any pressure on the buttocks, you have to lay on your stomach. But laying on your stomach all the time may affect the result of the breast surgery. So from this point of view staging is recommended for certain types of procedures.
2. The risks of long surgery are mainly related to general anesthesia. After 4 hours the risks of blood clot to the legs, or worse yet, the lungs, increase rapidly. Respiratory complications also increase. The good news is you can largely avoid these by choosing Intravenous anesthesia. Because you are breathing on your own and you are using more local anesthetic you are using less general suppression medications. This is good.
The concern about blood loss, especially in outpatient surgical facilities, where by law you cannot give transfusions, is also a concern. Luckily, use of Tumescent anesthesia can vastly decrease this risk.
For sure, see more than one Board Certified Plastic Surgeon before you make your decision.