45 days after the item is considered lost- checkout is 3 weeks, then several overdue notices. Essentially about 3 months. But you can also always renew aside from a few things.
In many places an overdue will automatically become lost after ~2 weeks - ~3 months. Then you'd be charged the cost of the item and sometimes a processing fee to cover the costs of preparing a replacement book. If you have a lost book, still bring it in, I would happily waive fees up until the point the replacement order was placed. Libraries generally just want the books back.