The option is there, but I'm not sure that it means delete the scan file if it is older than x days, but delete entries older than X days. I have mine set at 15 days,
I rarely if ever run on-demand scans (outside of tests for forum topics) as they are much depreciated on a resident on-access antivirus scanner.
That said I have my full scan log, and that is still there and the last scan was more than 15 days ago, but there is only that scan run on the 7th of July and nothing earlier. In theory that should be gone also, but it is the only one remaining.