As Sankung suggested, give yourself a couple hours. Also, bring a camera, the view is awesome!
I usually walk it at my leisure, meaning I really take my time. It usually takes me about two hours and that includes intermittent stops to look over the river in either direction or just to sit for a break and take deep breaths.
Depends on your pace
If you're walking at least 2 hours
I would say 2 hours minimum
Three and a half to 4 hours that should give you plenty of time to enjoy the scenery
About 3 and a half hours. It takes about an hour and a half to cross one way.
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