yeah the subsequent labours can be shorter.. but if you were induced with Synto, there is no way of knowing for sure how long your first labour would have been. My sister went through this on her second as her first was induced too. I can only say what Raven does - you really will know. I laboured at home with Haz until 9cms (only because the hospy was 500m up the road) and went in when I was getting the contractions on top of each other. But you really don't do that if you arent on their doorstep - takes a long time to walk like that, lol.

Just go in when you no longer feel comfortable without someone there. Always the best barometer is how you feel at the time.

Even ourselves can mess up our birth plan because for all the best intentions, we have to follow our own instincts at that time.