General Anesthesia after hair transplant - how long should I wait before having another surgery involving general anesthesia?

Hair loss from general anesthesia is very rare.  It is thought to be related to decrease in blood flow to the scalp for an extended period of time.   Perfusion to the scalp can decrease under anesthesia if blood pressure is low and/or if the scalp under some external pressure, like padding to hold the head in position.  In some surgeries, like spine surgery, you are face-down and there could be a fair bit of pressure exerted on the front hairline, which could involve the grafted area.   
For the first few weeks after a hair transplant the blood supply to the new grafts is still developing.  In theory, hair grafts may be at higher risk of injury from low blood flow during this time.   To be conservative, you could wait 6 weeks, at which time normal blood supply to your grafts has been established.   

Hi and welcome to our forum!
The hair loss experienced by patients under a general anesthetic is unusual.  Anesthesia itself (or any other form of stress) may cause temporary shedding of hair (telogen effluvium), but this condition usually spontaneously resolves.
Transplanted hair is quite delicate for the first week or two after transplantation. Patient positioning on the operating table for a second unrelated procedure may injure newly placed hair grafts. Speak with your transplant surgeon for clearance to proceed with the second procedure. We would wait at least several weeks.
Best wishes...