Is a wife's statement to her husband, "I am an infidel"—out of anger at his repeated disbelief in her faith—considered an act of disbelief (kufr), and what is the impact of that on the marriage, given that the incident occurred years ago?

A woman's speech is serious, but it is not considered disbelief (kufr) nor does it affect the marital bond if she intends to express her disapproval of him calling her a liar or asks him if he imagines her to be a disbeliever, especially since the context suggests she does not intend to describe herself as a disbeliever. As long as the possibility exists, the default is the persistence of faith, and acting upon the hadith, "Avert the prescribed punishments (hudud) from Muslims as much as you can," averting disbelief is even more paramount.