GarageBand '11: Set the project length
When you create a project, by default the project is 32 measures long. Projects can be as long as 9999 measures, in any tempo.
Each project has an end-of-project marker at the end of the project. The end-of-project marker appears as a purple triangle in the ruler. You can set the length of a project by moving the end-of-project marker. If you record past the end-of-project marker, or drag a region or loop past it, the marker moves to the right to accommodate the region or loop.
To set the length of a project:
Scroll the timeline to the right so that you can see the purple end-of-project marker in the ruler.
Drag the end-of-project marker left to shorten the project, or right to lengthen the project.
When you share or export a project, the exported file extends from the beginning of the project either to a few seconds past the end of the last region or to the end-of-project marker, whichever comes first. If the cycle region is turned on, the exported file matches the length of the cycle region.
If you export a project containing a movie, the exported file cannot be shorter than the duration of the movie.