How long shall I wait before proceed with eyebrow transplant after scar formed in eyebrow?

More info needed. Please provide more information as to the cause for the eyebrow loss alongwith photo documentation. Treatments are dependant on many factors and are best provided by a plastic surgeon who can review the history and examine the patient.
Depends... There are many factors which need to be considered before any surgical intervention is planned. The type of injury, the time elapsed, the quality of the the recipient area and other factors must all be weighed. The best answer is wait long enough to reduce the factors likely to cause failure and maximize the chances of success. An in-person consultation is necessary for an accurate answer.