Every toddler is different, and afternoon naps will slowly stop when your child is ready to stay awake for the whole day. To an extent, a toddler's napping habits depend on how he or she sleeps in the night young children need around 12 to 24 hours of sleep a day, and if more of that shut-eye happened at night, less will be likely to take place during the day time.
Statistically speaking, half of all kids still naps at four years old. That means that naps might extend beyond the toddler years, and into the preschool or even Kindergarten years. Most five year olds don't need an afternoon nap any more, but there are some that will still take naps even at that age. If your toddler has suddenly stopped napping, only you can assess whether it's because he or she is truly ready for it. some toddlers get really cranky if they don't have a nap, while others do just fine. If your toddler stopped napping, you can still practice a daily quiet time, during which you read books together and do some quiet activity.
If you are hoping to start naps again, you can try by being consistent about nap time, and perhaps reading a book in bed or taking a drive in your car to encourage sleeping. My children both stopped napping completely at around two years old, and nothing good convince them to go back to napping. In the end, we met their sleep needs by doing our bedtime routines and going to sleep a little earlier in the evening.