Assuming the most efficient manner for extraction, and a ready supply of other necessary materials not mentioned herein, and given the current estimates about the volume of Earth's ocean, how much energy (in Joules) could be extracted via fusion, given the deuterium in the ocean?
Some rough estimates (you can dig up more accurate numbers): The oceans contain about 321 million cubic miles of water (source: http://oceanservice.noaa.gov/facts/oceanwater.html), or 3.5e20 U.S. gal.
1 gal seawater contains roughly enough deuterium to provide the same energy as 300 gal of gasoline (maybe slightly less - that's the part for your homework!), so the oceans are equivalent to 1.1e23 gal. gasoline.
Conversion factors: 1 gal. gasoline = 1.24e5 Btu; 1 Btu = 1055 J; 1e15 Btu = 1 quad; U.S. annual energy consumption is a little under 100 quad; world annual consumption is about 500 quad.
So, the oceans contain about 1.3e28 Btu = 1.4e31 J of fusion fuel, which is 1.3e13 quad, which is enough to supply energy at the current rate of consumption for 26 BILLION years. (Compare: the earth is about 4.5 billion years old now. It is estimated that it will become too hot to sustain life in about 1.75 billion more years.)
So the answer to your question is: AN AWFUL LOT!!