An affirmation can be repeated whenever you have a spare minute throughout the day. However, most people like to set a time aside where they will sit down for 5 or 10 minutes to carry out their daily affirmations. This means that they are far less likely to miss any days. I would suggest five minutes in the morning before work or just after breakfast, and then five minutes in the evening before bed. This ensures your subconscious mind has a chance to work on them while you sleep.