3 nights is about right. 2 days to check out their attractions leisurely( you have to buy a book of tickets that allow you to visit a certain number of attractions ), 1 day to join a bicycle tour ( there's a free tour conducted by a bunch of students. Excellent way to see the countryside), and maybe 1 day to just hang around and chill, or join a food tour.
Hoi An is a very relaxing place, with cheap delicious food and 1-for-1 happy hour drinks. In the evening, the riverside is alight with traditional lamps, very scenic.
So if you want to chill with your viet gf, it's as good a location as any in Vietnam. If you rush, 2 nights is also enough, but 3 nights will allow for a much more relaxed pace.