The short answer is once the studios (led by the AMPTP) and the union agree to a fair contract for everyone. That could be completed in a week—SAG-AFTRA leadership has said they are ready to return to the negotiating table—or it could stretch through the fall.
The best guess right now is that it will last until Labor Day, which would impact the film release calendar for 2024 and the TV shows networks air through early 2024. A work stoppage going to September could set Hollywood back by six months. Right now, with no one at the negotiation table, it seems like the strikes will last until at least September.