Currently up to 20 log files are retained, 5 MB in size each. To keep log files longer, there are a few options. They are controlled via the service configuration file, which (depending on your installation) can be found at:
C:\Program Files (x86)\Complete FTP\Server\CompleteFTPService.exe.config
Look for the <log4net> section, and then the RollingFileAppenders.
You can do a number of things here:
1. Increase MaxSizeRollBackups to a number higher than 20
2. Increase the MaximumFileSize
Alternatively, you can
3. Change the RollingStyle from Size to Date, and add a DatePattern, e.g.
<param name="RollingStyle" value="Date" />
<param name="DatePattern" value=".yyyyMMdd" />
If you are more adventurous, you can investigate other log4net configuration options.
Please note that the CompleteFTP service will need to be restarted after making any configuration file changes.
Note also that all files in the C:\Program Files (x86)\Complete FTP directory are overwritten each time you install an update, so you'll need to reapply your modifications after installation.