I'm curious about this too. I have heard anywhere between 6-9 months, but don't take my word on that! My DD is still napping 4 times a day at 4 months.
we dropped the third nap around 5 months. she takes one nap at 830/9 and one nap at 130/2. she goes to sleep at 630/7, STTN and wakes up around 630.
my rule of thumb is I leave her in her crib for at least 1.5 hours. she doesn't cry when she wakes up, even if her nap is short. she usually falls back asleep, but sometimes she does;t-i think the down time is good for her. she usually sits there quiet and sucks her thumb, or babbles and rolls or falls back asleep. if her first nap is short (45 minutes) then her last nap is 2-2.5 hours. or she takes two 1.5 hour naps.
like i said she STTN so it seems to be enough sleep for her.
Little Riley-our first little girl coming March 1st, 2013 (or sometime around there;)
Re: Dropping a nap