Quick answer: Never.
Tatoos Damage Your Genes”
Long answer. Leviticus 19:28 You shall not make any cuttings in your flesh for the dead, nor tattoo any marks on you: I am the LORD.
In addition, no cuttings and no balding. Leviticus 21:5 They shall not make any bald place on their heads, nor shall they shave the edges of their beards nor make any cuttings in their flesh.
If you want to make a pile of money, open up a tattoo removal parlor. In 10 years bunches of people are going to fall all over themselves to pay bigga-bucks to get your services.
Titanium dioxide, used in tattooing to transfer color, has been found to affect both genes and also a possible cancer-causing material. Gene damage can affect future pregnancies. See these links. TiO2 does not release from the body but is stored in the tissues.