When is the best time to visit Shanghai? At least how many days need to stay if i m bagpacker?

The spring from March to May is recommended as the best time to travel Shanghai. It is warm and pleasant. Spring in Shanghai is from March to May. Spring flowers in bloom make the city lively and attractive. The temperature is comfortable around 20 C (68 F). Light rain happens however will not disturb your trip. Long-sleeved shirts, thin sweaters or coats are suitable for this time.
As a bagpacker, I think 3-4 days are OK for you to visit Shanghai.