You could hit all the ones on the national Mall in a week, but unless you're gonna blow through them it would probably take up all of your time every day. They're free so it's not like you're losing anything by leaving after an hour if you don't like it. If you want to see all of them you could prioritize what exhibits are unique/semi unique to the Smithsonian. Like the Natural History museum is very cool with a bunch of great fossils, but a lot of it is also not that different from any other natural history museum you've been too.