As far as I understand it, there is not a time limit for having an open plan if the treatment is on-going; however in the real world we try not to have treatment plans which span over months because it is not financially favourable. If the patient is returning for treatment every couple of weeks then you have no choice but to keep the plan open; if the patient has an exam appointment and then doesn't return for 3 months to have the treatment then you have the option of closing the original plan and starting afresh as there may have been changes in the patients oral health status over that time period.