This varies a bit by system - please note there are never fines on digital books. As checkouts are on a timer, you can never have an overdue book.
Cloud Library eBooks: 21 day checkout, please return book if you finish it early; can have 5 books checked out at a time. No renew feature, but you can checkout a book again immediately if no one else was on hold for it.
Enki Library eBooks: 21 day checkout, can return early; up to 10 checkouts at a time. No renew feature, but can checkout a book again immediately if no one else is on hold for it.
Freading eBooks: 14 day checkout, it works by a "checkout allowance" that gives you 5 checkouts per week. Unused checkouts roll over for up to 4 weeks. If book expires before you finish reading, just download a new copy.
Britannica E-STAX: no actual checkouts, use as many books as you like. There is an optional download of books for offline use, those files will expire after 2 months, but you can redownload immediately if still using the book.
Digital Storybooks (BookFlix): no actual checkout, no limits.