The default is to keep reload logs for 30days and the server automatically cleans them up.
You can change this behaviour by editing one of the config files, make sure you have a backup of the file first and restart your QDS service afterwards:
File should be located in c:\Program Files\QlikView\Distribution Service\QVDistributionService.exe.config
Then you can edit the file and change the following section to a larger value
<!-- The number of days to keep QDS logs --> <add key="NbrOfDaysToKeepQDSLogs" value="30" />
Also the Event/Performance/Session logs should be persistent but might not have the type of information your looking for.