I'm sort of an expert for time it takes, having been there several dozen times
Be there before it opens - I think the gate still opens about 0800, although I haven't been there since April and with sequesters and all one never knows.
Anyway, get NLT 0830 if possible to give yourself at least 30 min to look at a and photograph all the outside stuff. Once you get inside, go straight to sign up for the bus to the annex so - it has a schedule and need to know which one you are on - assuming it is open again since the sequester. They give you only 45 min at the annex, so you need to hustle for photos and looking. I take few photos over there now, since I've shot them all - I now look for new stuff and reshoot those which didn't come out as good as I likes.
You can photograph all the aircraft in the main buildings within about 3-4hrs, but that doesn't give you time to stop and look at everything. So if you have a short time, just look and shoot the planes. If you have time to spend a couple days, then you can spend a whole day in each section reading all the stuff on the walls and examining the displays.
You certainly want at least one whole day if you are short on time passing through, but even if you get only half a day, you can go through quickly to look at all the planes.
If you have a few months prior to going, sign up for the restoration facility tour - they fill up fast. That will take a couple hours out of you main museum time. I did my first tour there in April.
Doug's right, though - weekdays and as many days as you can allot will give you the full experience.
Oh, and I've never done the IMAX - not my cup of tea. So I can't speak to that.
My first trip was in 1969, the summer before my senior year in high school and we had just moved back to Ohio after 5 years in Colorado. It was still at Patterson Field. Went again the next summer just before going in the Army, and didn't visit again until March 1972 after it moved. Then my last leave before getting out, in August 1974, I again visited it. Once I was back to civilian status the end of Feb 1975, I was over there about every six weeks to see what was new (I lived about an hour away). That was standard visit schedule until leaving Ohio in Sept 1978, and then from the Chicago area we were down there at least twice a year until Dec 1995 when we moved to Iowa. Now it's only once a year, and a couple years we didn't get there at all.