I have a question about waves and particles. Since waves and particles seem to be somewhat interchangeable at the subatomic level, at what wavelength does the wave / particle duality stop?

Albeit tiny, spherical DNA structures like these are much larger than the buckyballs that have exhibited wave/particle duality in experiments.
Image Credit: Cornell University

That's a very good question — scientists are trying hard to figure out the answer! We don't actually know yet where the wave/particle duality stops.

In theory, this duality never completely disappears. All objects behave like waves to some extent, no matter how big they are. But because of the rules of quantum mechanics, the larger an object gets, the harder it is to do an experiment to test its wave/particle duality.

Objects with a large momentum (either they are massive, or moving fast, or both) have a small deBroglie wavelength, making the design of a measurement to elucidate their quantum behavior very difficult. You can see an example calculation here to determine the quantum mechanical wavelength of a baseball.

Since that wavelength for a baseball is so much smaller than even the wavelength of light, it makes measuring that wavelength nearly impossible. Conversely, a particle with a wavelength that corresponds to microwave frequencies (roughly on the scale of 10cm) would have a very tiny momentum.

For instance, an electron traveling at the slow speed of 1 centimeter per second would have a deBroglie wavelength of about 7 cm. Scientists are continually searching for larger objects that still exhibit quantum effects.

Thanks to such research, we now know that objects as big as biological molecules or buckyballs still exhibit wave/particle duality.

Perhaps by the time you're a famous scientist, we'll have seen evidence of quantum effects in even larger objects!