We could use nuclear for 100% of our energy needs for the next 5 to 10 thousand years (assuming everyone in the world uses as much energy as people in the US and a world population under 20 billion.) If we switch to fusion we would have energy well into the billions of years range.

The problem with fusion energy is that nobody got it to work yet. Unless you count solar power.

I've heard other numbers about the nuclear energy, but anyway - time will tell.

Uranium is vary common. On average you could extract more energy from coal by using the trace amount of fissionable material in coal than you get by setting the stuff on fire.

(http://en.wikipedia.org/wiki/Nuclear_power) As of 2004, nuclear power provided 6.5% of the world's energy... At this point, the enriched uranium, containing more than the natural 0.7% U-235, is used to make rods of the proper composition and geometry for the particular reactor that the fuel is destined for. The fuel rods will spend about 3 operational cycles (typically 6 years total now) inside the reactor, generally until about 3% of their uranium has been fissioned, then they will be moved to a spent fuel pool where the short lived isotopes generated by fission can decay away. After about 5 years in a cooling pond, the spent fuel is radioactively and thermally cool enough to handle, and it can be moved to dry storage casks or reprocessed. As opposed to current light water reactors which use uranium-235 (0.7% of all natural uranium), fast breeder reactors use uranium-238 (99.3% of all natural uranium). It has been estimated that there is up to five billion years’ worth of uranium-238 for use in these power plants.[33]

PS: The five billion year figure makes some assumptions about the rate of use but once you start looking into the numbers it's far enough into the future that our predictions are just about meaningless.

Edit: There are a lot of odd regulations and political history around whey we do it this way but long term supply is not the real issue with fission.