Some toddlers get the knack in a few short days, while it can take others a year or more. But if you miss the signals or don't wait for their cues, it can take six months or more. It makes it easier, too, if he can pull his clothes up and down by himself. Steer clear of overalls if he can't undo the clasps. Physical and mental readiness aren't the only factors involved.
Motivation is key.
If your child demonstrates a desire for independence and shows an interest in imitating others' bathroom habits, it may be an auspicious time to plunge into the process. Children may figure out how to have a bowel movement before they learn to urinate in the potty because they typically only have BMs once or twice a day, usually at the same time. Keep a relaxed and positive attitude, and encourage your toddler to get accustomed to his potty. Let him help pick the potty out at the store, then let him play with it, look inside, sit his teddy bear on it. If he wants to take a rain check on this, don't push it.