Activation, (to my understanding) depends on the material you are trying to activate, as well as your prior knowledge on the subject.
If, for example, you've got a history with a subject activation will be quicker (in my experience).
If, you've never dealt with a subject before it might take longer to activate and gel.
The more complex the concepts the longer it can take as well. It's best to activate in small chunks (as well as the rest of the steps).
Tracey