A 3rd cousin shares a great-great-grandparent, and a 4th a great-great-great-grandparent. Humans are all distantly related, the furthest is 50th cousins. Marrying a 3rd or 4th cousin is legal (along with 2nd but that's still weird), and its not socially unacceptable. Also its very common historically, and even today.
I've never even met my 3rd or 4th cousin, and I think that's when things get distant.
I would do my 4th cousin (probably cause I don't know any of mine, while I know some of my 3rd and most of my 2nd), but that's as far as I'm willing to go inbreeding wise.