A water flosser does not need to be used twice a day. The current guidance from the dentists is that you should brush your teeth at least twice a day and floss once. Of course, you can use the flosser twice a day if you wish, but it is not necessary. If you only floss once a day, you should make sure that it is right before you go to bed. That way, you can clean off everything that might be stuck on your teeth from the day, and go to bed with a completely clean mouth.