Modern Principles of Shaping - 6. Keep Training Continuous!

I'm writing a 10 part blog post where I discuss the Modern Principles of Shaping by Karen Pryor, to help solidify them in my mind.

6. Keep training sessions continuous. The animal should be continuously engaged in the learning process throughout the session. He should be working the entire time, except for the moment he’s consuming/enjoying his reinforcer. This also means keeping a high rate of reinforcement.

1. Continuous engagement can me either that the learner is always doing something or getting CT.  OR it could also mean that the learner is always eager and paying attention, even if they aren't actually doing anything.  The latter situations is what I strive for, and sometime hope to accomplish so I can capture some behavior  
2. Working the entire time means that the animal should respond as best they can to what they think you want them to do.  However, if something is on queue, and they choose to do something different, they're still engaged.  
3. Training time is valuable to both the teacher and learner, so be sure to set the expectation that the reward source is available now.  
4. A high rate of reinforcement could be as many as 20 treats per minute.  That seems tough to do when the learner is enjoying their reinforcer.  But maybe that means my treats are too substantial....
5. "High rate" is subjective, so some subjects will be able to do more with a lower rate of reinforcement.  
6. Reinforcement (and the information conveyed by the click) is the key to successful training.  So the more C/T you have/minute, the more likely you are to see change and success. 

Outstanding Questions: How many times should you repeat something?  Should the same criteria be used for multiple rewards in a single session?  Or is switching between criteria every C/T OK?