Depends what you mean by "go into labor" lol.
With DD, my contractions started at about 10:30 a.m. and were 3-5 minutes apart from right then, all the way up until she was born at about 10:30 p.m. that same day. I had not been having any contractions except for the stray BH every now and then prior to her birthday.
With DS, I had been having WEEKS of prodormal labor, and contractions that HURT and felt real but they were random. Then sometime mid-morning the day before he was born, I was having contractions that didn't really physically feel any different, but mentally I sort of "knew" that they were real contractions. But they were about 90 minutes apart.

They picked up in frequency to 5-7 minutes apart at about 6:00 p.m. and then he was born at 2:00 a.m.
So I'm going to say that my labor started at 6:00 p.m. w/ DS and 10:30 a.m. with DD.
