With my last two, it was about two and a half weeks. They were regular, strong, and would get progressively closer together. Then around 2-3 min apart they would suddenly slow down and stop. All the normal tricks wouldn't stop them. It was a good thing we lived close to a hospital, I wouldn't make the call until they were 1-2 min apart.
Wife to N, Momma to M (8/26/09), J (4/21/11), and H (1/2/13)