Q: How long as does an automatic gearbox/ transmission last ?

Most automatic transmissions these days are pretty trouble free for about 120 to 150 thousand miles. After that all bets are off. I have seen plenty of cars go well past 200 thousand without problems. If you have one of the CVT transmissions that Nissan is making, that is relatively new technology, so there might be issues earlier than with the conventional units. As for the noise when it shifts, there really shouldn’t be anything noticeable. But it might not be in the transmission itself. There could be a problem in a motor mount or a CV joint that is making your noise. If you want to have that checked at your convenience, contact YourMechanic they can send a technician to your home or office to check out your Nissan and let you know what it is going to take to solve it.