When should I use a laser comb after hair transplant, and is it effective?

Although laser combs have received FDA approval, this is only
for the issue of safety, not effectiveness. There is no strong and convincing
evidence to support their usage across all individuals who wish to grow more
hair. Growth after a hair transplant is primarily contingent on the doctor’s skill. This
includes factors like:

(1) selection of the right follicles which have not yet been
affected by miniaturization

(2) storage of the grafts at the appropriate temperatures

(3) avoidance of air drying as a possible risk for dessication,
the leading cause of follicular death

(4) careful handling of the follicles and preventing injury
through surgical instruments.

Oxygenation is also important for the follicles to survive.
However this can be limited if the blood vessels are constricted due to certain
medications (e.g. asprin) or chronic smoking. 

You can use a laser comb but it probably will not help.  Laser combs are safe to use but its efficacy is controversial. 

First, consult with the physician who performed your transplant. He or she will be able to give you specific feedback on when to use a low level laser therapy (LLLT) device post-op, like the LaserComb product to which you are referring. LLLT devices have recently been in the news after a new study showed a positive correlation between the use of a LaserComb device and new hair growth. As quoted from our Institute's summary of the study:"In a study of 334 men and women, a popular LLLT comb device was found to
stimulate all-natural hair growth after just 26 weeks of use. The test
group consisted of 146 men and 188 women, but the results were not
gender-specific. Both men and women experienced natural hair growth as a
result of the LLLT treatment." Again, we strongly urge you to seek the guidance of the surgeon who performed your transplant before making a decision.

First and foremost you should check with the surgeon who performed your procedure when he or she recommends using the comb.  Most of the time, use with 7-14 days is fine but stick to your surgeons's adviceThere have been no studies yet on the efficacy of low level laser AFTER a hair transplant. Anectodal evidence (sharing of information between surgeons) does suggest it speeds up regrowth. 

There is no definitive proof that LED light will help your transplant, but there is also no harm to be done. In general LED light helps to stimulate a wound healing response, decrease inflammation and possible stimulate enhanced  metabolic activity at the hair follicles.  I would suggest waiting 10 days before starting with the comb so as not to disrupt the hair follicles. Just check with your transplant surgeon.