The best time to listen to subliminals is when you are in a relaxed state of mind. Right before going to sleep at night or during your sleep are perfect times to listen to subliminals.
You can use them almost anytime during the day when you do not need your full concentration, but they are much more effective when you are calm and relaxed.
The two best times to listen to subliminals during the day is when you first wake up in the morning, your mind is still calm, and right before going to sleep.
They are still effective at other times during the day when you are doing other things.