I had complete FFS and BA. It was an extensive operation. From the first demerol push, when I lost consciousness, through to recovery was 10 hours. The actual time I was under general anesthesia was a bit less than that but not much.
During that time you must be attended by an anesthesiologist or physician. Use of GA shuts down respiration and major body systems like digestion. Only a physician would be able to give you a definitive answer because too much rests on the patients overall health.
When I had SRS, I was under for about six hours.
It is the surgical team's responsibility to determine the total GA time based on patient health and the number of procedures to be performed. Generally it's better to do as much as possible in one session rather than endure the stress of multiple operations under GA.
-Sandy