Assuming no history of inbreeding and little/no chance your future kids will inbreed, 1st cousin is generally safe as a one off. It raises the genetic risks a bit but not enough to be a red flag. Obviously there are present day social mores to be concerned about.

The further you go from that the better, about 5 steps is probably sufficient though (closest shared ancestor is your great-great-great-great grandparent).

But really the problem with inbreeding is repeated incidents, as you are narrowing the gene pool significantly. This is a concern in more insular communities (like the Amish) where you don't get a lot of people from outside adding to the gene pool.