Roughly every 7 years for most of the body, major organs tend to take longer, more like 10. Your brain replaces cells at a way slower rate, you’ll only Theseus your brain about 80% at best if you live a long healthy life.

Thats not the point though, 390 mg is a lot even for a healthy adult.

Tom Paris tells the Doctor holograms are just light and aren’t real matter, the Doctor turns and slaps Tom.