According to some quick googling and rough math, there's about 5.5 billion years worth of U-235 present in the Earth's crust on the top 15km. If we consider that we can maybe reach 0.5km down, (deepest gold mine is 4km), and assuming it's evenly distributed, then that's only 180 million years!! (2024 global electricity usage)

Think we can figure out breeder reactors in 180 million years? If we're going all nuclear, I'd expect them in under 1,000 years, but I'm not an expert.