Prentke Romich Company
http://www.prentrom.com/support/article/206

How much Recorded / Digitized memory does my Pathfinder Plus hold?

The Pathfinder Plus containing a 256 MB card has around 4 hours of Recorded  (digitized) memory.

The Pathfinder Plus containing a 128 MB card has over 60 minutes 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.

The Vantage/Vanguard Plus with a 256 MB card has around 4 hours of Recorded (digitized) memory.

The Vantage/Vanguard Plus with a 128 MB card has approximately 30 minutes of Recorded (digitized) memory.

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.