Here is a look into the debates over how long a short film is supposed to be.
The divisions between different film forms is not quite as firm as most people think it is, but it is still there. The feature film business is what people tend to think about when they think of professional filmmaking, even independent filmmaking. For many new filmmakers, the short film form is going to be what gets them started, and it can also be a specific type of film that you may want to do for your concept or story. When it comes to distribution, film festivals, and press information, you are going to have to be able to identify with the rest of the canon of short films. The real question then is how long is a short film, and the answer varies a little bit.
Length in Context
How long a short film is depends on who is making it and where it goes. The usual answer to the "how long is a short film" question is around twenty minutes, ranging from fifteen minutes to twenty two depending on the venue. This answer is not going to be too important for most situations unless defined. For example, a film festival will usually set its own limitations for how long a short film can be in that category, as well as how long a feature film must be. This is also going to be set by the broadcaster that you may be showing the film on, as well as the filmic categories for awards.
In more open short film festivals you are not going to have the same limitations on short film lengths since all the films are going to be in the short form, so the maximum may be set upwards of thirty minutes. If you are shooting for DVD distribution, or online distribution, then you are also not going to have as strict of limitations over how long the short film must be. Some awards categories will actually allow a fairly long length for short films, and the Academy Awards will offer a total length of forty minutes including credits to the short film category.
Choosing the Length
A more traditional definition of how long a short film can be is between one and fifteen minutes, though oftentimes films under three minutes get a special category as long as they are not animated. If you want to be considered for the greatest number of film festivals and categories then you will want to place your film's length between ten and fifteen minutes, as long as it can maintain a full story structure in that period of time. Forty to forty-five minutes is usually going to be the limit for any short film category you are going to run to, and you may even be able to find feature length categories beginning at this length. Most feature categories will start off at an hour, so if you feel as though your film is at the tail end of the short category then you may want to go for the extra length. This is going to be much more practical if your film is still in the screenwriting or pre-production stages.