Flossing once a day is ideal. Typically before sleeping. The reason being, flossing often increases the gap between the teeth causing a bad bite. And flossing just before sleeping takes away the entire day’s plaque and food particles.
I floss every evening after brushing my teeth to be sure that no food particles, that bacteria can feed on, that sit in my mouth overnight.
You should floss thrice a day. One of those times should be a thorough flossing where you get a little below gumline all around each tooth. the other two times should be following major meals to make sure you remove any food that got stuck
Ideally, you should floss after every meal but I sometimes don't get to do so but I try to floss at least once a day – at the end of the day.