Time spent with a drill. I ofter change my routine to fit my feelings and how I am at the moment. The time set for each drill though should be enough for you to learn from and not too much so you are begining to repeat yourself. It is not a matter of chalanges but of learning. If for example, you do a drill where one hits and the other evades and strikes on the same step/breath/body turn than you can continue learning from just that for a long time. It is good to introduce continious work or more types of hits but it is the basic understanding of your movement which is growing. Not a set of moves. Think for example of the internet guros which specify a fixed set of exercises that will set you free, make you strong as a tiger in 44 minutes and you will never want anything else again. They pose a lesson too. If you believe something that does not come from your experience than you are blind to the world in some way. Go ahead and make a list of nifty exercises that need to be done in a sequence and only you can sell this piece of paper. Does that make sense ? By doing the work we grow and by doing good work we grow up.