I can't remember when my first DS dropped his AM nap... I want to say around 18 months. DS#2 is 12 months and still going strong with the AM & PM naps.
My DD did at around 18 months. Once in a while, she'll take a short morning or afternoon "cat nap" for about 20-30 minutes in addition to her long afternoon nap (2 hours)
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DD is 21 months, and at about 17 or 18 months, she went to only 1 nap per day, and it is usually between 12pm and 2 pm, and last from about 1 1/2 hrs to 3 hrs, depending on how active she has been.
I have a feeling this is out of the norm, but on the weekends, my 21 month old still takes 2 naps (9-11 am and 2-4 pm). She usually goes to bed about 8 pm and waks up around 7 am. At daycare, during the week she obviously only gets one nap a day from 12-2 pm. I think part of it could be that she has pretty bad allergies and is on allergy meds which can sometimes make her drowsy.
Re: When do toddlers typically drop their AM nap?