I was travelling for the last two weeks, so I missed the Superman release.
We are now two weeks later, and all the IMAX theaters around me have switched all of their showings to Fantastic Four. I was expecting a split between Superman and F4, did WB somehow messed up contractually?
IMAX runs in general tend to be short, because the theaters need to make room for the next movie showing in IMAX.
I think only Christopher Nolan has that kind of pull in the industry to demand exclusivity to IMAX. It sucks, but the only guaranteed way to see IMAX movies is on opening weekend (or hope for a re-release).