About 1/16th of an Inch
The tattoo needle used to insert dye into your skin should drive about 1/16 of an inch into the skin, or through the first three layers of skin. A good, experienced tattoo artist will make sure that the ink is penetrating deeply enough into the skin to be permanent, but not so deep that the flesh is damaged and scar tissue begins to form.
Traditional Tattooing Techniques
Some of the traditional tattooing techniques, such as Samoan and Thai techniques, are not as subtle as the modern tattoo machines with their rapidly moving needles. In the traditional techniques, the tattoo artist uses a single needle shaped from a thin stick of bamboo to inject the ink into the skin, one strike at a time. The depth the needle reaches during this process is less consistent, and these techniques can result in scarring.
Most modern tattoo parlors and artists use a tattoo machine, or "gun," to inject the colored inks rapidly into the skin. A conscientious and qualified tattoo artist will work to keep the needle to a consistent depth of about 1/16th of an inch, or no deeper than the third layer of skin.