Assuming the internet has been growing exponentially since the beginning and will continue to do so indefinitely.
IPv4 has 32 bits and lasted us 34 years so far. Let's call it 32 years. Address space would need a nice and tidy 1 bit extra per year. That would mean IPv6 would last until around the year 2110.
But will the internet keep growing exponentially until then? How many people will even be alive by then? According to https://en.wikipedia.org/wiki/Projections_of_population_grow..., predictions used by the UN range from 7 to 18 billion people. Let's round it up to 20 billion people. How many IPv6 addresses could we give out to each person? According to https://www.wolframalpha.com/input/?i=2%5E128+%2F+20*10%5E9, 1.7 * 10^28, which is a lot. But okay, we have a lot of computers per person and not all computers are associated with individuals. So what if we do something crazy, like give out IPv6 addresses to the (human) cells in everybody's body? https://www.wolframalpha.com/input/?i=1.7+*+10%5E28+%2F+cell..., that would be about 1.7 * 10^14. What if we give each cell some IPv6 addresses per second (pretending all of our cells live for 80 years)? https://www.wolframalpha.com/input/?i=1.7+*+10%5E14+%2F+(80+...
Here we land on 67 thousand addresses, per second, for each human cell in the bodies of 20 billion people, for the span of 80 years.
That's how many IPv6 addresses we'll need to use before we run out.
We have made this mistake before, but 128 bits really is a lot.