As a native, I have been to this places so many times. I would not suggest a best time to go, as each season you would see a little different charm. In winter, there are far less people, and it is cheaper too. I suggest you follow a tour guide (a lot English tour guide available) to enjoy more of the scene. Two hours would be sufficient.
A small bonus: the free Suzhou Museum nearby. Less than a minute walk, it's the Suzhou Museum. Huge collection of ancient Suzhou, which was one of the main cities and economic centers in China for a long time.
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