Number of days after 40 weeks to assume induction:
Get probability of labour starting in N days:
Statistics copied from this site. Caveats are that it's American and a relatively small and self-selected data set taken from an ongoing survey. It turns out it's really hard to find data on this based on days rather than weeks! Number of days at which to assume induction means that data only considers spontaneous labour up to that point and inductions after that point. There is no adjustment for time of day or how long labour takes.