Two days is the minimum in my book. The time you need also depends on what you want to get out of it. Sure you can buzz through all of the tables in a day but can you dig through binders, boxes, etc. looking for the gems you need? I can't.
Also, for me the National is a social event and an opportunity to catch up with collectors / dealers I only see once a year and talk about cards. That takes lots of time away from browsing too.
Highly recommend you spend more than one day there.
jeff
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