apparently after 12 hours infection can set in. because ds2 wasnt born untill arround 18hours after mine broke, i copped a huge ear bashing of one midwife and both ds2 and i had to have antibiotic drips and blood tests every few hours for a few days "just to be sure" due to the time frame.
At the time, They told me 24hrs but that was 15 years ago. Proberbly is 12 hrs by now. I did know it was for the risk of infection though. This was just one of the many things the hospital / OB (at the time) stuffed me around on. I had to stay in hospital the whole time though & didn't get any infections... Luckily!