FYI, did you know that IMAX is Canadian. The original IMAX theatre, The Ontario Place Cinesphere, is right here in Toronto.
When I was young, only 40-minute documentaries were shown in the format and at the Cinesphere they were usually shown as an 80-minute double feature.
It was only within the last 5 years or so that feature-length films started being released in the format (though before that, it was common practice to show a 35MM film -- IMAX is 70MM -- on the IMAX screen, which results in a short of "letterboxed" IMAX - i.e. the picture only covers the centre of the screen).
EDIT: I just took a look at the
Cinesphere website, and the 40-minute documentaries that were playing when I was a kid are STILL playing.