Depending on the level your students have and their interest in the subject matter, your students will generally talk for 2 to 5 minutes per card.  (C1+ more, A2 students less.)  

As you usually have 15 cards per set, the SimplyConversations take between 30 and 75 minutes of class time. 

However you should also factor in the pre- and post-task activities you will do and depending on how many/what type of errors and mistakes you will have to review, feedback can take between 10 and 15 minutes.

On average each set lasts 1.5hrs. 

Sometimes your students will take control of the subject in the middle of the "game" deviating the conversation onto other topics - much like in a natural "mother-tongue" conversation - this is absolutely allowed and encouraged (okay, within reason), as it is a sign of them gaining fluency.

When this happens, the best thing to do is to simply save the remaining cards for another lesson.