Prentke Romich Company
http://www.prentrom.com/support/article/212

How much Recorded / digitized memory does my SpringBoard Plus Hold?

The SpringBoard/SpringBoard Plus with a 256 MB card had almost 5 hours of Recorded (digitized) memory.     The SpringBoard/SpringBoard Plus with a 128 MB card had almost 1 1/2 hours of Recorded (digitized) memory.

Typically, 1 minute of digitized memory is equal to 1mb (megabyte) of storage space on your memory card. 

IMPORTANT NOTE!   We recommend that you use a Secure Digital Card to store song files (MP3, WAV, or WMA) instead of using your internal flash card, since songs require so much storage space.  There is a slot for a Secure Digital Card on the top of the Pathfinder next to the Memory Door.

You should also consider the format of the sound files you import.  WAV files require the most space. The same song in an MP3 format will use less storage space. The WMA format typically requires even less storage space.

Note:
Wav file, short for "Wave" file by Microsoft.
MP3 file, short for "Moving Pictures Experts Group audio layer 3".
WMA file, short for “Windows Media Audio®,” by Microsoft.