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Naturally occurring uranium is >99% the U-238 isotope and <1% the U-235 isotope. The U-235 isotope is the isotope required for nuclear activity. The amount of time required to enrich uranium depends on the size of the enrichment facility and the level to which you want to enrich the uranium. There are two basic enrichment categories, LEU and HEU. LEU is low enriched uranium where the U-235 content is in the single percentage digits. This uranium is strong enough for nuclear power plant usage. HEU is high enriched uranium and has a U-235 content at 80% or higher. This is the amount required for use in nuclear weapons.
Keep in mind that a nuclear power plant uses on the order of metric tons of LEU enriched uranium per year (1 metric ton is 1,000 kilograms). A large enrichment facility can produce this amount in about 1 month. So producing a single kg of LEU would only take on the order of days. But producing 1 kg of HEU would take considerably longer (I cannot find any information on the time length; perhaps not surprising since weapons grade information would be classified). Keep in mind when talking about a uranium weapon, tons of enriched uranium is needed so 1 kg isn't very much (the Hiroshima bomb contained 64 kg of HEU and is tiny in comparison to modern weapons). Also, these enrichment facilities contain thousands of centrifuges or diffusers (the most common methods for separating uranium isotopes).
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