Don't count on it and then it's more likely to occur. That is what spontaneous activation is about.
When you get to a point and can tell that you need nothing out of a book or need to activate a book I personally consider that spontaneous activation. We then move to bringing the information to the level of consciousness that we need it at. Guess what? That happens with just about every book.
Finding yourself talking about something and not realising that you learned it from a book that you PhotoRead isn't going to be that uncommon. The problem is without manual activation how do you know that you learned the information? It's a bit of a paradox so there are probably people experiencing spontaneous activation without realising what has occured so one will never be able to give you accurate figures of when, or how long it took for the first spontaneous activation to occur.
Alex
