We moved to one nap at 14 months, after two days of first morning nap refusal. After 3 days of one nap, massive OT set in and I went back to two naps.
For the next month we did 2 naps except on the odd exception when we again had a nap-refusal for the first morning (short) nap - but instead of deciding this meant a permanent move to one nap I just let it slide as a one-off.
At 15 months we went to one nap, with every 4th or 5th day being two naps. There were lots of early bedtimes on days when the one nap didn't work (an OT nap b/c I should have done 2 naps or a UT nap because I put him down too early). Now, at 15.5 months I am pretty confident that we are onto one nap, although he is very low on A times and high on sleep needs when he is teething, and so I have done the odd day with 2 naps to catch up from that - and on these days both naps tend to be short (30/45 + 45/1hr) and an early bedtime.
HTH!