ChatGPT agrees "To determine the long-term annual sales of phones for a company with a constant customer base of 1 billion, where each customer upgrades at different intervals, you first need to know the average replacement interval, T years. The annual sales can then be calculated as 1,000,000,000 divided by T. For example, if customers upgrade every 2 years on average, the company would sell approximately 500 million phones per year. This approach assumes the replacement intervals are consistent or average to T years across all customers."