Happily, most parents seem to know that already, doctors at the Children's Hospital of Philadelphia found. In their study of 378 parents of toddlers, the average age when intensive toilet training was started was 28.7 months.

"For those children starting toilet training before 27 months, the process took a year or more, but if they were started between 27 and 36 months, it took five to 10 months," says Dr. Nathan Blum, a developmental pediatrician at Children's Hospital and lead author of the study. The research appears in the April issue of Pediatrics.

The optimal time for speedy toilet training, the study found, was when children started training just shy of their third birthdays. It took five months to train them if they started between the ages of 33 and 36 months, Blum says.

For the study, Blum and his colleagues followed the parents of 17- to 19-month-old babies, interviewing them every several months to track their babies' toilet training until it was completed. The authors defined the beginning of toilet training as when the parents first took out a potty chair and started initial discussions. Intensive toilet training was defined as asking the child to use the potty more than three times a day.

The researchers found that while starting training earlier than 27 months wasn't harmful to the child -- there was no increase in constipation or withholding stool -- it took longer than if the training started when the children were older.

The average age when the children were toilet-trained was 36.8 months, with the girls completing toilet training, on average, sooner than the boys, at 35.8 versus 38 months, respectively.

Dr. Marcia M. Wishnick, a New York City pediatrician, says this study provides nothing new.

"It is generally accepted that most toilet training takes place between two and three years," she says.

She says there are ways to assess when your child is ready for toilet training, including asking parents how many diapers a day they change.

"If they're down to four-plus diapers during the day, we know the child is using sphincter control," she says. "Also, if the child is communicating in some fashion that they don't like being soiled, we know that the development is there, and it's time for a parent to take a pro-active role in toilet training."

Once that time is at hand, she says, toilet training happens quickly, "from one week to three or four months."

Blum adds his study aims only to provide "rough guidelines."

"This study suggests a range where people should be looking for optimal toilet training, but if you think your child is ready before 27 months, or, on the other hand, if you think they're ready at 3 years old, then do what you think," he says.

More information

The American Academy of Pediatrics offers some basic guidelines for toilet training. For further information on when to know your child is ready for toilet training, go to the Nemours Foundation.