It takes about 6 to 8 weeks for a human fingernail to grow completely from the base to the tip. So, that's how long it takes for a nail to be "replaced" by new growth.
However, if you're talking about replacing a nail that has been lost due to injury, it might take a bit longer for the nail bed to heal and for new growth to start.
Let me know if you had something else in mind. 😊