Laser light therapy post FUE. How long can it be used for?

It is best to ask your surgeon about post operative care as each doctor has their unique regimen. 

LLLT is an FDA approved treatment for hair loss. Its effect however is typically subtle and minimal. The results would be lost if LLT is discontinued. 

As far as we know Low Level Laser therapy is a maintenance adjunct-thus it should be used as long as you would like to maintain your hair.

The laser can be used as long as you wish as it will not cause damage to the new grafts over time. The laser probably will not help the growth. 

Light therapy is one of those maintenance treatments that can definitely improve your hair growth.    These devices ALL speed wound healing.  When you stop the treatment, the benefits stop too, unfortunately!
Dosing for LLLT (also known as PBMT or photobiomodulation) is likely to be best in the 668-684nm wavelength range for 20-30 minutes every day or every other day.  Stationary light sources like caps work best.  Your head does not develop a tolerance to light therapy so no need to cycle the treatments. An optimal device would also ensure that the light actually GETS to your head, but other than straight shaving the area you want to treat, no device is yet on the market which will solve that problem!  The good news is that enough light seems to be getting to patient's scalps that hair is growing, and we have data from the 1960s on LLLT/PBMT so there does not seem to be a limit to how long you can use it for.
Good luck!