Every library is different. I have two library cards through Libby. One of them gives me 3 weeks and another gives me 2. If no one else is waiting for the book, you can renew your loan.

Also, if you are using your Kindle, you can put your kindle in airplane mode and continue to read even though the loan has technically expired. It goes back to the library but since your Kindle is disconnected from the internet, it stays on the device until you connect it back to wifi.