It really depends on the woman. I was induced with my first, but I recently had my second. Contractions started Tuesday am (I was already 3cm dilated and 80% effaced according to doctor's appointment day before). They weren't regular that day, but at about 1 am Wednesday morning they were every 8-10 minutes apart and lasting 2 minutes. I was 5 cm and fully effaced when I went into L&D (at 1:30 am). By 2:15, I was 7cm. By 3am I was 10 cm and having the urge to push, had him at 3:20 am.
It was much quicker than I expected especially for a spontaneous birth like that. But I wasn't sure entirely what to expect since I was induced before. But as said each woman and labor is different.