Just doing some pencil and paper math here. I was originally wondering if Bitcoin ever were to see the type of transaction volume that Visa sees, say, something like 300,000,000 transactions per day, would the lack of available bitcoin addresses ever be a problem. If a new address were to be used for every transaction, then it would still take roughly 2^40 days before we used all of the 2^160 addresses. I obtained this number by dividing the total number of bitcoin addresses (2^160) by the number of bitcoin addresses created per day (300 million), which comes out to roughly 2^40 addresses. So even without expanding the number of addresses, we won't have to worry for another trillion years.