Refer to the help article from eBook Central on how copy/print/download limits work. In summary, most eBooks reset every 24 hours. Some eBook's options for copying are somewhat limited, for example, only 5% at a time (or about 15 pages) can be copied. If an eBook is not resetting after 24 hours, you may need to log out and log back in and/or you may need to delete your browser cookies.
In addition, be aware that some eBooks have limited user access, or we may have only one copy, so you may have to wait for another user to finish using an eBook before you can access it.
Refer to the articles for more detailed information:
Ebook Central: Per User Per Day Copy, Print, and Download Limits