The scans are displayed on the scan schedule page and run every day at 11pm on my server. They will generally start within an hour of the scheduled time (to avoid overloading our servers). Yours might be different but you can tell by the schedule below the option to enable them. The times and days will be greyed out but one box will be checked.
The premium version, among other things, will let you choose when and how much to scan. So, lets say you get middle of the night traffic only. You could set your scans to run during the day.
Does that help?
tim