The half-life of carbon-14 is 5715 years. How long will it be until only half of the carbon-14 in a sample remains?

The half-life of any sample is, by definition, the amount of time it takes for half of it to degrade.

It's an exponential decay, not a constant rate of decay; after one half-life, the remaining half becomes the "input" for further decay, so only half of that half decays during the second half-life, leaving you with one-quarter; similarly, after three half-lives, you still have one-eighth, after four, etc.

It should be noted that carbon-14 decays, but the nitrogen it produces during that process may also go up through the atmosphere and react with cosmic rays to produce more carbon-14, an example of a cosmogenic nuclide. For this reason, carbon-14 has been present on Earth for billions of years, even though its half-life is only a few thousand years.

Only half of the carbon-14 in a sample will be left after 5715 years.