You're basically asking how to convert assets (market cap) to income (GDP). But there isn't a fixed conversion: it depends on your discount rate (how valuable is future money compared to current money) which itself you can loosely think of as related to interest rates, which themselves change over time. Alternatively, think of the P/E ratio, which also changes over time. There's no bound. If you think humanity will exist forever, and you're willing to wait forever, and you think Apple will continue selling iPhones at a large profit forever, its value is effectively infinite.