My question is: Can I extend the garageband file limit any longer than 2 hours? Does compacting the file extend time limit? Or do I need to arrange for breaks every 2 hours during the seminar to restart a new garageband file?
It would be safer, to keep the file sizes smaller than 2GB. There has been no definitive answer from Apple with respect to the File Size limits in the current GB version, afaik.
The only reference I could find was valid for GB '08, and it may still apply: http://support.apple.com/kb/TS1040
GarageBand stores the audio files in the the GarageBand projects as aiff files, and the aiff standard restrict the the file size to 2GB. If Apple respects this standard, an audio clip cannot be longer than 2GB.
And how is the drive formatted, where your projects are stored? If the drive is not formatted MacOS X extended, the filesize limit would be built into the filesystem. But Mac OS X extended disks should not have this limit.
Are your projects on a drive that is daisy-chained? Then this could be the problem: "File size limit reached" error when recording to daisy chained drive