Most drop to 1 nap by 18 months. That's about as specific as it gets.
You let them sleep for as long as they want. If a child is sleeping, there's a reason for it. I can't tell you the number of times my daughter has taken an extra long nap, with me wondering if I should wake her and then discovering she has a cold the next day. Her body knew it needed the rest.
Keep in mind that an overtired toddler will not rest well at night. Forcing them to drop a nap or forcing them to wake up early from a nap will actually disrupt the night time sleep. They will have a harder time getting to sleep, staying asleep, and may wake earlier in the morning.
In the 13 to 18 month range, my daughter was taking 1 nap - usually about 1 hour long - and her bedtime was 6:45. She woke in the mornings about 6:30. By 24 months, it was 7:15am, 2-3 hours long, and 7:15pm.
at 5:39 PM on May. 6, 2009