here is how mine went...
1st labor: 15 days early- 15 hours
2nd labor: due date- 8 hours
3rd labor: 11 days late- 7 hours (induced)
4th labor: 8 days early- 9 hours (water broke)
as you can see, mine really have followed no pattern. i guess you could say all of them after the first have been quicker. dont know if thats how it generally works, but thats how mine have come