Strictly speaking, the fuel needed for nuclear fusion is limited because the Earth has a finite size. However, the simplest fuel that can be used is deuterium, which is the same as hydrogen except that the nucleus contains both a proton and a neutron instead of just a proton. On Earth deuterium can be found in about 0.015% of water molecules. Even though this is a small percentage, there is a lot of water on Earth. If industrial fusion reactors can be made to yield more energy than they consume to run them, then a large reactor would probably consume on the order of around a kilogram of fuel per day. Given there are more than a trillion kilograms of deuterium on Earth it would take a very long time to consume it all, and additionally some of the reaction products could also be used as fuel.