How long after the hair implant, I can start using Minoxidil 5%?

It is best to ask your surgeon about post operative care as each doctor has their unique regimen.   Minoxidil is not necessary for transplanted hairs to grow.

After
a hair transplant we ask that patients wait 7 days before starting minoxidil.
However, if by some chance you still have open wounds and excessive scabbing,
hold off for a few more days until after the wounds have closed.

I usually tell my patients to wait a week before resuming minoxidil, though as you can see from the other answers different surgeons have different opinions.  You should check with your surgeon for his/her preference.