When they can go on their own, when they just do it.. you dont have to ask them every half hour or hour. I agree totally that accidents happen though. My son it potty trained, goes all day bt still wears a diaper at night just in case. It is usually dry in the morning though.
My son takes off his diaper when its wet, but shows no signs that he is ready to even try. I did an experiment to see how far he was and i put underwear on him and showed him where the potty chair was and told him if he needed to go to let me know and i can take him to go, well he just went in his underwear, i hope that my little guy is ready soon because he starts pre school in aug.