default: 24 fps
By default there are 24 animation frames in a second, therefore the default 250 frames equal to 10.42 seconds (250 divided by 24). There are proposals to make the length of the default blender file equal to 240, in order to have an exact duration of 10 seconds.
To know how many frames you need to create an animation of a given length, just multiply the duration (in seconds) by 24, and change "Frame End" accordingly.
e.g. 1 minute = 60 seconds → frames = 60 x 24 = 1440
4 seconds → frames = 4 x 24 = 96.
other frame rates
24 frames per second (24fps) has been the standard for films since the 1930s. With modern technology, it's now more and more common to see video shot at 30fps or 60fps.
The frame rate can be changed from the Render panel in Blender up to 2.79 (in the Output panel in Blender 2.8), under "Frame Rate":
Since blender animations are frame-based, changing the frame rate will change the time length of the animation, while the keyframes will remain bound to their original frame. The frame rate should be chosen before starting an animation project.