It depends on what you want and how comfortable you are! If you want to do good stretching, toning of muscles and want workout like a resistance exercise you can do it slowly. ( around 90 seconds for finishing 12 postures).
If you want to advantages of Cardio, do it quickly. ( around 40/45 seconds for a full round of 12 positions). [Generally, beginners would have started sweating by this!]