What is the maximum amount of time between a bone graft and dental implant?

all depends on the type of bone used. Some bone grafts last longer than the others. But as a general rule, the regenerated bone will resorb ( collapse) if no implant is placed. The function of jaw bone is to hold a tooth.  If not  stimulated by tooth or dental implant, it will resorb the longer you wait, the more resorption.usually dental implants are placed within first year after bone grafting.  

Ideally implant placement is done after 4-6 months of bone grafting. Waiting longer then this can cause that bone to resorb.

after 3 months , is the best time to proceed to place an implant, because depending on the type of bone graft, it can reabsorb, and after 3 months you will have the body automatic healing process done, so if you are thinking about placing an implant, this is the best time, the only difference is if you got sinus lift , and bone graft, this is a different story.