Yes you're supposed to wait for them to expire. It's called spaced repetition/leitner system. You review at the most optimal time based on the memory curve. Please read the introduction to the website.

http://kanji.koohii.com/learnmore.php

Here's the idea: You want to effeciently memorize thousands of things (first 2000 kanji, then about 10,000 Japanese words eventually). Now, if you learned each just once, they'll eventually get forgotten. You also can't review all of them everyday either. It's a chore to organize the items manually to review at set times too. Plus, some you memorize better than others.

Taking all that into account, you are letting the computer make all the above decisions for you. Its "thinking" for each and every card is "has enough time passed since the last time you reviewed this card based on how often you've gotten this card correct to show you this card again".

So yes, wait for the cards to become expired to review them again.

"You'll soon learn to curse the 80 or so a day you end up being assigned."

Soooooooooo true!