Ballpark (Hours) that hair grafts can survive while awaiting transplantation?

If the conditions are right, a graft can survive well over 12+ hours. If not, less than a minute.  It takes experienced staff members to keep your graft alive.

Proper cooling and hydration are required for adequate survival, but the best time-frame is 8 hours max.Best.

The survival starts to taper off after 4 hours then dramatically more so after 8 hours.   A lot of these studies are done on FUT with much less graft manipulation so many FUE clinics tend to use lower cut offs to get optimal results.

8hrs is a good rule of thumb.  That is certainly within the framework of a grafting procedure, even if it takes all day.  The more critical factor is to ensure that the grafts are kept hydrated.  If they dry out, they won't last long at all!  

There is a classic study that show that if the grafts are left out of the body for less than 8 hours with proper cooling and hydration, the survival rate will be very high. For each hour extended, the graft survival reduces by 1% which means that if you put them back 24 hours later, only 76% will survive