A few of mine naturally gave them up around 2. I stopped one around that same age because he was not going to bed at bedtime anymore and was staying up way too late. Some kids are more tired than others and require more sleep. But by age 3 I don't worry about naps anymore- a quick nap in the car is sufficient, so I don't plan around naptime like I do when they are very small. We still do have quiet time in our house though- 1 hour of playing quietly alone. Sleep, read, play- I need it and they need it.
But in short, I would stop them when it starts to interfere with going to sleep at night.