I spent just 5 hours there, took only the basic bus tour, and felt like I missed half of what there was to see. You really need at least a whole day--more if you want to experience some of the paid extras.
You really need the whole day.
It depends on your interests. Saturn V / Apollo and Spaceshuttle Atlantis take about 4 hours.
There are restraunts there if you wanted, but it's up to you, we spent all morning and afternoon
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