hi there
my lo is almost 13 months and has always needed slightly more than the average daytime sleep for her age. our routine looks like this:
6.30am wake
9.30am nap 30 mins
10am wake
2pm nap 90 mins
3.30pm wake
7pm bed
You will know when she is ready to start the transition as she will either fight her pm nap and won't go down for it, or fight bedtime, or wake up at stupid- o'clock in the morning (like 4-5am) ready to start her day (that's what happened to us!)
Then the thing to do is start to shorten her am nap by waking her, to ensure she still gets a good pm nap. We went from 90 mins, to 1 hour, to 45 mins, and now have 30-40 mins max. I have to wake her every day from the am nap.
If she still sleeps soundly 11 hours at night and clearly needs the daytime sleep to then I wouldnt' be in a hurry to make changes
k x