Zara was around 12 months. Hugh was a super champ at 9 mo, but then we went away for a few weeks, so he did 2 short naps for that trip, then when came home at just over 10 months, straight back to 1 nap. He throws in the odd two nap day if there is something weird going on (like today) but that is uncommon. Zara's transition was completed by 13 months, although she did get OT on nap initially - but was fighting napping like no tomorrow, so we had no choice but to roll with it.
I agree that 2nd children tend to transition sooner, particularly if they are not kept home for their naps...Hugh was/is dragged to activities every morning, so it wasn't easy for him to nap until we got home...and those activities are essential for us in amusing a spirited DD! He loves the activities too now.