Because radioactive decay is a statistical process, we need some way of measuring the probability of this happening if we want to use or account for this decay. We could just state a straight-up percent chance of decay in a given period of time for a single atom, but this info isn't necessarily the most user-friendly in practice.

The "unit" that was chosen to represent how quickly a radioactive material decays is called half-life. Half-life is the amount of time it is expected to take for half of any amount of material to decay. Half-life is useful because it's constant for any amount of the material. The amount of time it will take for half of 100 grams or half of 1,000,000 kg of the material is the same.

For U-238 in particular, the half life is about 4.468 billion years. So, given any amount of U-238, assuming no material is added or subtracted from the system, we expect half of it to have decayed to Th-234 after 4.468 billion years. (And Th-234 also decays, which produces something else that decays, and so on until some stable element is produced.)

If you're curious how to calculate decay rate from half life, there are plenty of great resources online (wikipedia included). I'd post the equations here, but I have no idea how to format equations on Reddit.

Feel free to ask any follow-up questions if you have any interest in nuclear decay or nuclear engineering.