What is the best time to have juices?

It is best to have juices empty stomach in the morning, 1-2 hours before breakfast. This is important because a juice must be given sufficient time to do its work (whether that is detoxification, or nourishment). If you eat food immediately after a juice, the juice cannot do it's work properly. 

For any reason if you cannot have juice in the morning, evening (about 2-3 hours after lunch, and 2-3 hours before dinner) is also a good time to consume it. 

In case you do yoga/exercise in the morning, you can have it before or after your exercise.