Well, you can record until the 4gb runs out!
There are several factors to recording size that will affect how long you can record. The range is probably the biggest factor. The larger the range setting the more bytes that are recorded per ping. Also, if you have the 83khz transducer enabled. A file is recorded for each transducer, so having it enabled will add 33 percent more data to the recording.
Typically 4 gb SD card can record for hours at say a 100' range setting. I would suggest breaking it up in to 30 minute recordings if possible. If you plan on playing it back on the computer, most softwares right now cannot handle very large files.
Hope this helps,
Robert