How much is it from 798 Art District to Nanluoguxiang subway? How long would it take @ ~4pm?

If by taxi, the fare is CNY 40 to 50 in the day time. And the duration is about 20 to 25 mins.

Or you can take subway line 14 at South Wangjing Station, get off at Jintai Road Station. Then transfer to subway line 6 to Nanluoguxiang Station, Exit E.
The ticket fare is CNY 5 per person. It takes around half an hour on the way.