A pretty up-to-date BMJ abstract
As blueviolet recommended, gentlebirth.org is a wonderful resource. This is a snippet from the site--hope it helps--btw, BMJ is British Medical Journal

:
Emergency Cesarean As Accessible for Homebirth As In Hospital
Safe interval for emergency caesarean section is 75 minutes
BMJ* 2004;328 (20 March), doi:10.1136/bmj.328.7441.0
When the decision to deliver by caesarean section is made, time to intervention should be less than 75 minutes to avoid poor maternal and baby
outcomes. Thomas and colleagues (p 665) analysed 17 780 singleton births by caesarean section in Wales and England in 2000. They found that,
compared with deliveries completed within 15 minutes of the decision for an emergency caesarean section, mothers' and babies' outcomes did not differ from deliveries within 75 minutes. Babies delivered after 75 minutes were more likely to have an Apgar score of less than 7, and their mothers were more likely to require special care. Never the less, the target of 30 minutes should remain as the benchmark for service provision, the authors say.
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Women laboring at home within 20 minutes of a hospital operating room have the same access to emergency surgery as women laboring in that same hospital.* Most surgeons are not physically present in the hospital while their clients are laboring; they are either at their office seeing patients, home sleeping in their beds, or simply going about their business.* (Many doctors instruct the nurses not to call them until the baby's head begins to crown.)* Whether an emergency arises at home or in the hospital, it still takes time for the surgeons to be paged, to drive to the hospital and to arrive at the scrub sinks.* It takes time to assemble the anesthesiologists, nursing staff and neonatal teams necessary for a cesarean section.* Hospitals offering labor and delivery services must be able to start cesarean surgery within 30 minutes of being notified of the need, which gives women laboring at home plenty of time to get to the operating room ahead of the surgeon.
How can this be safe?* Childbirth "emergencies" usually develop over the course of hours rather than minutes, especially when they're not actively caused by interventions such as artificial rupture of membranes, administration of pitocin, spinal/epidural anesthesia or pulling on the umbilical cord to deliver the placenta.
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What is a reasonable time from decision-to-delivery by caesarean section?* Evidence from 415 deliveries shows that fewer than 40% intrapartum deliveries by caesarean section for fetal distress were achieved within 30 minutes of the decision, despite that being the unit standard.
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Two papers and an editorial in this week's BMJ describe the difficulties in meeting the target of 30 minutes between the decision that an urgent caesarean section is necessary and delivering the baby.* [May, 2001]
Interval between decision and delivery by caesarean section -- are current standards achievable? Observational case series
"Conclusions: The current recommendations for the interval between decision and delivery are not being achieved in routine practice. Failure to
meet the recommendations does not seem to increase neonatal morbidity. "