It depends on which eBook you are downloading. Most downloadable eBooks will provide a time frame for downloading. If you do not see this, please email back and let us know which ebook collection you are looking at.
However, if you have internet access, there is no real need to download the eBook. You can access it as often as you need.
The best eBook databases we have access to are eBook Central and EBSCO Academic Collection. Feel free to contact us if you're having issues access an eBook.