A good rule of thumb is that 250 MB can store about 266 minutes of audio. However, the amount of audio that can be stored in 250 MB depends on the quality of the audio files.
Quality of compressed audio files is commonly expressed in bitrate, a measure of the storage used for a given track length. Typically, bitrate is expressed in kilobits per second, or kbps. Audio files are commonly compressed to 128 kbps, which affords a compromise between sound quality and storage space. Apple iTunes defaults to this bitrate, at which a minute of audio takes up 7,680 kilobits of storage space. There are 8,192 kilobits in a megabyte. At lower and higher rates of compression, 250 MB can store more or less music, respectively.