There is a known issue with longer projects. The issue varies, depending on the type of audio file you export, the Auto Normalize setting, and which version of GarageBand 4 you are using.
To change the Auto Normalize setting:
- 

- Choose GarageBand > Preferences. 

- In the Advanced panel, set the Auto Normalize option. 

GarageBand 4.1.2 has the following export length limits. If you try to create an export that exceeds the length limit, a warning dialog will come up:
File | 
Max. Length | 

---|---|
16-bit AIFF, Auto Normalize On | 
1h 41min | 

24-bit AIFF, Auto Normalize On | 
1h 41min | 

16-bit AIFF, Auto Normalize Off | 
3h 20min | 

24-bit, Auto Normalize Off | 
2h 13min | 

AAC | 
None* | 

MP3 | 
None* | 

*16 or 24-bit AAC or MP3 files with Auto Normalize on that exceed 1h 41min will not have updated media previews. 16-bit AAC or MP3 files with Auto Normalize off that exceed 3h 20min will not have updated media previews. 24-bit AAC or MP3 files with Auto Normalize off that exceed 2h 13min will not contain media previews.
GarageBand 4.0 - 4.1.1 has the following length limits. If you attempt an export that exceeds these lengths, there will be no warning, and the resulting file will not be usable:
File | 
Max Length | 

---|---|
16-bit (any type), Auto Normalize On | 
1h 41min | 

24-bit (any type), Auto Normalize On | 
1h 41min | 

16-bit (any type), Auto Normalize Off | 
3h 20min | 

24-bit (any type), Auto Normalize Off | 
2h 13min |