The point is to be consistent and to not abandon the schedule when you don’t feel like writing.
Of course, not everyone can manage to write 7 days a week. That’s why you can also decide what “daily” means to you: perhaps it means 3 days out of 7, or on weekends, or 4 evenings out of the weekdays. The point is to be consistent and to not abandon the schedule when you don’t feel like writing. How long should you write for? Suggestions vary (15 minutes according to Bolker and Jensen; 30 minutes according to Hayot) but the goal is to write in manageable chunks of time that you can sustain and replicate regularly.