I thought so too, but it seems reasonable considering we're 70% water, much of it flowing and actively taking stuff out and around. Plus cells are apparently way more dynamic than is typically conveyed in high school bio (I think there was a quantamagazine article a while ago). So it's perhaps surprising that a body's atoms aren't fully recycled even faster than that.
The Ca in bones is probably renewed very slowly. C H O N very fast, in particular water. Na, K and Cl are very soluble and also get renew very fast. Some rare elements like I are probably reused a lot and renewed very slowly. I'm not sure about P, my hand waving is confused. Assuming a burger per day, we ate our weight in meat every few years, but I'm not sure if all the Fe is absorved in and exchanged for the internal one.
