I would floss every time you were to brush your teeth, which for me is once in the morning and once at night before I go to bed. I always eat before I brush my teeth in the morning so I have that clean feeling before I go out and face the day but if you brush your teeth before you eat I would recommend flossing afterwards to get all the trapped food out. I will always floss before I go to bed though because it’s a very good habit to have since the amount of saliva you produce in your sleep goes down.
Yes, imo it's best to floss ever time there's something stuck between your teeth. When in doubt whether you feel something/ when you don't feel anything ever, you might floss after Every time you've eaten something, snacks included.