H.R.
Both of my children were about 18 months when then shifted. However, the shift might need to be inconsistent for a while. What I mean is this: do two naps for a day or two and then do one nap...back to two...then one...for a few weeks. There may be days that going to one nap just doesn't happen because she gets too cranky. It doesn't need to be all one or all the other, at least for a few weeks. Then try to shift it to more days with one nap.