Hi, the rules say
When you expend a card you must always, if possible, place it on the top row of the Card Display. If the top row is full then you place it on the bottom row.... Only one card can be placed in a space... If both rows are full and you need to expend a card then you remove and discard all of the cards from the top row.
So not as soon as its filled?
EDIT - beaten to it. The discarding of cards is clear, but I'm still unclear on my original question.