The way we describe it in the book is a "type" of action. So you need to at least delineate some kind of specificity. With that said, we think its one of those things where if players try to cheese it, the system's boundaries keep it in check since you still need to roll to counter.
As for frequency of adding new cards, we intentionally left that to the player. Certain strategies are going to require more cards than others, and at certain times players are going to realize they wanted the deck to do something different then how they built it. With that said, there is always going to be a point in creating cards where you're deck will get too big to see things you want to consistently, and at that point the players goals will change.