There is no set standard. On average, an animated short film can be around 10 minutes, but festivals accept both shorter and longer films; however, if your animated short is more than 40 minutes, it is then considered an animation feature by some festivals.
At the same time, if your film is longer than 20 minutes there may be animation festivals which will not accept your film; longer animation shorts are more difficult to programme, especially at festivals with a small competition, where the number of sessions is limited.
On the other hand, if your animation short is very short (up to 3 or 5 minutes), there are festivals with specific competitions for very short films and an associated programme. There are also festivals where a narrow theme (genre, technique) is set.