Well, being specific:
Auto updates occur while browsing, so the time really doesn't count.
Scan for viruses every day = 10 minutes.
Scan once a week using Spybot, 2 to 4 minutes?
Clean out the tifs, cookies, history, deleted files, recent documents, maybe 30 seconds.
So we are looking at less than an hour per week, in little 10 minute increments.