There was a time when I thought my DD was ready for 1 nap but it ended up just being teething. I think that was around 15-16 months. However, she is now 21 months and I am just transitioning her to 1 nap a day. But she has always been high sleep needs. I just rode it out, and she ended up doing 1.5-2 hours in the morning and 1-1.5 in the afternoon until just now. On days where she was extra tired she'd do 2 hours in the morning. It's hard to say how much sleep they need. I've heard 13 hours is the minimum, but my daughter needed 15. I think every child is different and you just have to go with what their needs are. My daughter used to wake up around 7 and go down for her first nap at 10am, the second around 2:30pm. Then 7pm bedtime. Not sure if this helps or not, but I do remember thinking, "Oh no, her naps are getting shorter. Please don't tell me it's time for just 1!" But I kept a consistent schedule, and there were days where one nap was really short, but it wasn't until she started refusing afternoon naps that I went with the 2-1 transition.