I chose scan weekly and indeed read the text below the setting. My gripe is that NMS scans way too often for my taste nevertheless. The frequency is never only once a week. I didn't count exactly but it wouldn't surprise me if its daily with sometimes even twice a day.
I also experience the same issue, the scan takes 10 minutes during which you cannot use the tablet. It has got so bad I have replaced Norton with another security app.
By default the app seems to scan when the device is first turned on. It will run LiveUpdate to ensure the latest malware definitions are installed, then a scan will be run with the new definition files.
The Weekly setting seems to be a fallback in case no other scans are done in that time.