Requests for books that are currently used by somebody else have an expiration date of 3 months. This means that when you are queuing for a book with several people before you, you might be removed from the queu before you get the book. You will then have to request the book again.
You can see the expiration date by logging into LUBcat. Click your name on the upper right and you will get to "your summary". Click "Holds" and find the book in the list. In the column "Expires on" you can then see on which date your request will expire.