You can change the duration of time Growl messages appear on screen by going into the growl preference pane specifically the "Display Options" section.
Is it possible to somehow make the max duration longer than 10 seconds? I want to use this as part of a display, and want the notifications to hang out for around an hour before fading.
I managed to find this. First, choose the display style you want to change, and modify the duration to something other than the default. Then open up com.Growl.GrowlHelperApp.plist and adjust the corresponding duration parameter. Works great.