The standard amount of time is 1-2 weeks, however it can last much longer and depends on how the audience responds to it. For example we have just had a short film out with the re-release of 'Get Carter' and the audience enjoyed the short so much that the cinema asked if they could keep hold of it to show with another feature. Through the initiatives like The Cinema Programme, short films get picked up by distributors to be released infront of feature films. Optimum Realising and Metrodome being good examples of short film friendly distributors. Short filmmakers that make 35mm prints of their films are doing so because they want to show them in cinemas.
Cinema is escapism, TV is reality.
A filmmaker is trying to tell the audience a story that transports them to another time, place, and world. Those stories can be ten minutes or ninety minutes, as long as the story is told well and entertains an audience will watch. It's a spontaneous response. Most of the films we see have 35mm prints available so there is no problem getting films into the cinema. The most common problem is not prints, but matching the appropriate short film with the feature film.