Having visited Zhouzhuang a few times, I think 1 day and 1 nightis a good amount of time to spend there. If you can't spend the night, 1 day is enough
The scenic area is quite small, you can walk all around in in 2-4 hours. Obviously if you take it slow and spend more times in the shops there, it will take longer
Also, keep in mind that it takes 1.5-2 hours to get here from Shanghai by bus each way (there's no train service to here), so if you do a day trip here it can be a little rushed
The reason I highly recommend spending the night is that the number of tourists go down a lot in the evening and morning hours, giving you more space to enjoy the town in peace. Also, the vibe of the town is very nice and cozy at night when everything is lit up. Worth checking out!