It used to be that you could check out a book for three weeks, then renew it (online) for three weeks, up to 8 times total. Now it's still technically for three weeks at a time, but all the renewing is automated. You just have to keep track on whether someone else has requested it, because if they have, the renewing doesn't happen. It's very handy! New books usually have to go back after the first three weeks, but most books can be kept for the whole 24 weeks.