According to an estimate I once saw, fusion using Deuterium and Tritium (the two "best" hydrogen isotopes for the task) can supply mankind with electricity for about 2000 years, considering we extract all the deuterium from sea water and the tritium from lithium.
After that, the world's oceans will be depleted of deuterium and the next closest source of it is.... the sun (or maybe Europa).
After that, the world's oceans will be depleted of deuterium and the next closest source of it is.... the sun (or maybe Europa).
