I can’t imagine spending anymore than 3 days in Hanoi, unless you’re one of these people that wants to know every little detail of history about every single monument, building, and structure in every city you visit.  Hanoi is relatively small so it’s not as if you can spend days just wandering around exploring the city.  Even though it’s much quieter compared to Ho Chi Minh, it’s still pretty full on in the center around the Old Quarter and when you’ve been walking around for a few hours, you can really feel the pollution getting into your lungs (that’s why a bike ride of the city is not a bad idea).

There’s plenty of flights into and out of Hanoi from Hong Kong and most other places so it’s easy to get to.  You’ll enjoy the Burger King at the airport before you pass through immigration if you’ve been eating Pho for days on end.