I think it depends on your requirement. You could estimate and increase your log size but this may lead to the following issues:
If you have a lot of activity and the log file, you might end up having huge log files if you want to keep a whole 180 days
You cannot be sure that you will have the 180 days of logs with 100% certainty.
Perhaps a better approach is to automate the copying of the logs to another destination (lets call it the archive). You could then have more control and certitude that you will keep exactly 180 days of history.