The studies all examine cord clamping delays between 30 seconds and 5 minutes because it is a topic that has only recently been looked at in terms of research, and 5 minutes is a convenient time for hospital staff. The cynic in me says that longer than 5 minutes and the midwives and obstetricians might start getting bored... :P What you will probably find is that in the coming years, now that it has been established that 5 minutes is safe, they will start looking at research into the safety and benefits letting the cord stop pulsating naturally; although it must be said that the experience of thousands of independent midwives for hundreds of years says that of course letting the cord stop pulsating itself is safe.