The application logs are limited in term of how much data they can hold at directory level and as well as file level. Once that limit is reached the purge/truncation of the application logs will done by system. In case if you try to download the data purged it will show below error. The file size limit can also result in some overwriting or truncation.
Release : 6.6, 6.7 or higher
Component : CA RELEASE AUTOMATION RELEASE OPERATIONS CENTER
These properties are configurable and please refer section Execution Log Control in document for doing so