E.T.
Every kiddo is different, you'll know when she's ready. I think my son started the 1 nap/day right around his first birthday. Even though he's almost 2.5 years old now, he still sleeps 12-13 hours at night and takes a 2-3 hour nap during the day.
Once she starts complaining about going down or when YOU can tell she's clearly not ready, back nap time up. eventually, the 2nd nap will disappear.