Twice a day, and as needed. I also use a water pick. Take care of your teeth. The cost to fix them (and the pain associated with losing them) isn’t fun. I didn’t start being super proactive about my dental health until I was in my 30s. Definitely wish I had started earlier.
Don’t forget regular cleanings at the dentist as well. The more you go the less painful it will be in the long run. I recommend at least twice a year.