The limit is determined by the rights favorite talking point when it comes to federal overreach into things they support. That is, it's determined by the states.
The repubs need to keep focus on this! This is Americas top priority! Every American poll conducted about Americans most important priority has abortion restrictions as the number one item! Number two, of course, is helping Canadian companies to be more profitable
Roe v. Wade effectively established a time limit to before viability, which at the time was basically limited to the first two trimesters. However, judicial interpretation of Roe v. Wade has effectively removed that limit as states repeatedly try to further restrict abortion and the courts repeatedly strike them down. At the same time, viability has steadily moved up - I think I saw reference to a five month preemie (around 22 weeks!) surviving.
It needs to be said that the mother legitimately has rights even after fetal liability, so it's not an easy question, and anyone who says it is an easy question is lying or simply wrong. That abortionists routinely abuse those rights does not mean the rights themselves are illegitimate. That legal liability attaches to those terminating another's pregnancy against the mother's wishes also does not mean the mother's rights themselves are illegitimate.