If they are truly ready? A few days to a week or so.
With our oldest we tried when he wasn't ready. It was a two week break (he was at nursery school at the time) and we had a week and a half of frustration, with nobody putting more pressure on our son than he was. So we backed way off.
Maybe 6 or more months later we tried again. A few days later he was trained, at the end of the week we sent him to nursery school in underwear, and a few months later we ditched the night nappies because he hadn't had a single accident.
With our second it was easier to recognize he wasn't ready... Not ready... Not ready... Ready, done, never looked back, just like that.