There is 1.4E21 kg of water on Earth. 0.03% of hydrogen is deuterium, a suitable fusion fuel. H2O has an atomic mass of 18 and O has an atomic mass of 16, so Earth has 4.7E16 kg of deuterium readily centrifuged out of ocean water.

D-D fusion converts about 2% of mass to energy. E=mc^2. So we have 8.4E31 Joules of fusion fuel ready for us on Earth. We used 2400 TWh of energy last year. If we used this amount indefinitely then we would have 9.8 trillion years of fuel.

Bonus: deuterium depletion would have virtually no environmental effect.