The answer to the question “…when will it require another chest X-ray…” depends on the home health regulations and, in turn, on the likelihood of exposure to tuberculosis (TB). For example, if the likelihood of your contracting TB is high, given the population you work with, an annual x-ray would be reasonable, as the PPD has become useless. But if you were very unlikely to encounter active TB, the interim between x-rays might be several years or, if the likelihood of it is practically non-existent, repeat x-rays would probably not be necessary, although required, none-the-less.
Thank you for your knowledge in this field. Right now we require our x-rays to be done every 2 years along with our annual TB test.