How quick can a blood work test for lupus or ra go from constantly being negative to finally showing up as a positive?

Anytime.: Could show up positive at anytime. BUT the test alone does not make the diagnosis. These diseases are still diagnosed by clinical picture and supporting tests and studies. If your test has been consistently negative may want to consider testing less frequently rather than more frequently.

Check the Laboratory: Call the laboratory and make sure that their quality controls were within their established standard deviations on the batch of tests that your specimen was run and also find out any possibility of false positives for that particular method that they are using to do your test.