I would say at least twice a day! Anything more than that is still great because it keeps your teeth and gums continuously free from bacteria and backed up plaque.
I was told by my dentist awhile ago that I need to floss at least once a day and brush my teeth twice a day. But, I'm only brushing my teeth once for now until I gain the habit of flossing and then I'll do twice a day. So far I'm proud of myself for sticking to it!! 🙂