Inner lip tattoos can last from 1to 5 years depending on the amount of acids in the mouth. It varies from person to person.
Lip tattoos are iffy in terms of will it stay or will it fade away... Although, typically, inner lip tattoos fade, there are cases where the tattoo will be fine upon first application. But usually, you'll need to go in for a number of touchups if you want the tattoo to remain permanent.
As for the pain of a lip tattoo... Well, like any tattoo the actual pain will vary among your pain tolerance, but surprisingly, most people say that the getting tattooed in your inner lip is not as painful as other locations.
As for aftercare of lip tattoos, all you need to remember is basic mouth wash. You need to rinse your mouth before and after you get the tattoo, as well as after meals and after smoking. You want to hold off eating for a few hours after the initial tattoo, as well.
But, in general, lip tattoos should not be treated any different than any other tattoo. Make sure to find a reputable tattoo shop and a reputable tattoo artist. And remember that lip tattoos are like regular tattoos, in regards to the thiner the line and the closer the letters are together, the more likelihood that the tattoo will fade and become unreadable after time.