How much radiation can a person receive over a lifetime without any negative health effects?

Depends..: Radiation from the sun (uv) is cumulative and can lead to cancer later in life. Uv radiation can cause small changes to dna that will build up over time. X-ray radiation does much more damage to dna but cells will either repair the dna or die. So interestingly, uv radiation is more harmful long term.

No one knows: Impossible to answer. Background exposure that everyone gets at sea level is 3 sv per year. Airline pilots get an additional 1.5 sv per year with no increased risk of cancer.

Complex: Question. Depends on many factors. If you have been exposed talk to radiology physicist at your hospital.