Depending on what studio/what title, commitments on a major new wide release can be 2 weeks to a month or more. Major titles/filmmakers can get crazy (George Lucas really got intense with Episode I requirements demanding largest screen, etc)
However they can be week to week commitments sometimes for smaller titles, smaller studios.
If something bombs incredibly, sometimes theatres/theatre chains will shaft it even if contractually they aren't supposed to, this happens a lot with smaller studios and or other titles besides big giant Marvel openings. I'd imagine a lot of showtimes for other titles are going to be dropped for more showtimes of DUNE this weekend for example.