Suggest Dosage For Over-the-counter Omeprazole

It used to be, when taking OTC omeprazole, that if I missed a dose I would be symptomatic within 2 or 3 hours of the missed dose. Now however I am taking it by prescription, same dose, and my doctor won t prescribe 2 20 doses, saying the research doesn t support this. So, as an experiment I only took one dose today and have not had symptoms. What am I missing?