In Earth's crust the abundance of natural uranium is estimated at between 0.9 and 2.8 parts per million, with most estimates on the higher end (see a comprehensive table in wikipedia, or for instance "Deffeyes & MacGregor, "World Uranium resources" Scientific American, Vol 242, pp. 66-76 (1980)"). U-235 accounts for 0.72% of natural uranium, giving an expected crustal abundance of between 6.5 and 20 parts per billion (ppb) for that isotope. Uranium has been mined commercially from ores of 100 ppm; some deposits have uranium concentrations hundreds of times greater.
Earth's silicate crust ranges from 6 to 50 km in thickness; at least 10% of it is already accessible to mining, and there is little reason to believe the remainder will be inaccessible to future technology. Given the enormous energy release from fission, even at the average crustal abundance and within difficult minerals there should be net energy return from U-235 extraction.
With a total crustal mass of about 1.4x10^23 kg, we finally have a total U-235 supply of close to 3x10^15 kg.
The primary (thermal) energy release in a fission reactor per kg of U-235 is about 8x10^13 J, 215 MeV per U-235 nucleus, or slightly under 1 MeV per nucleon. 3x10^15 kg then gives us access to about 2.4x10^28 J of total energy just on the simplest U-235 fuel cycle - that represents 6x10^8 or 600 million years of present total world energy use.
There is even more energy available from other fission processes; in principle all that's needed are elements heavier than iron, though elements lighter than lead have little energy to give up through fission. Barium is the most common element heavier than iron, but lead itself and thorium and uranium are particularly abundant for their atomic numbers. Total crustal abundance of thorium, uranium, and other such heavy elements is about 1 part in 10^5, for a total resource of about 1.4x10^18 kg. At 1 MeV/nucleon from fission (which would require breeder reactors and chemical reprocessing at the least) the total energy available would be about 10^32 J, or roughly 2.5x10^11 years worth of present demand.