When can I start using ketoconazole schampoo 2% after FUE hair transplant?

You should not be using ketoconazole shampoo unless you were told to use it by your doctor.  You should check with your doctor.

I usually recommend resuming all topical medications and products after one week from an FUE procedure. Ketoconazole shampoo is used by some patients for it effect on the hair follicles that are prone to hair loss.  It is not always needed to use this shampoo after a hair transplant, but if it is recommended by your doctor check with him about the timing of starting it.

After 2 weeks when the crusts are off, you can use any reasonable shampoo. I usually do not recommend ketoconazole shampoo as it adds no value to the healing process

this is an anti fungal medication that is used for   very dry scalp or dandruff. It may cause a bit of irritation and itching, so I recommend not using it until you are healed which could be about two weeks. You can also ask your doctor for advise.