Is it possible to set the duration of a GrowlNotify notification?
You can kind of do this with Growl GUI, not the best approach, but should work:

With the Grown GUI you can set event duration per-style. You can also assign styles per application.

So, if you assign different styles to different applications, you can achieve the desired effect. But that would result in different visual styles for different applications, which may not be desired. You may also be able to duplicate styles (don't know how) and assign styles that are the same visually to different applications. Alternatively you may be able to create new styles based on existing ones. 

Again, not the best solution but should works.