Most ebooks will have a duration limit, expiration, or rental period. This information is listed with the purchase of the ebook. At the end of the time period, you will temporarily lose access until you renew your license.
If the ebook is only available for 180 days, and is listed as 180 days on the website, you are responsible for renewing the license at the end of the time period. If the ebook is listed as "unlimited", then you license will never expire.
You may submit a tech support ticket on shelfit.com for assistance with renewing your license. Please allow 3-4 business days for renewing your code although you may receive the code sooner.