Under the studio system, I believe that each made around 50-52 A-movies a year (plus numerous B movies and other programmers), so they generally had at least one new feature each week. How long they ran was up to the theater owner (some studios owned theaters so of course they only showed their company's movies). Big hits ran longer, and yes, they were often re-released because the cinema was the only place you could see these movies. Studios could make more money out of big blockbusters like "The Wizard of Oz". Once tv's were more commonplace, this didn't happen as often, but I remember "Ben Hur" and "Gone with the Wind" being re-released in the 60s.

You might want to post this question in a film sub where some film historians can give you a more detailed response.