The Final Cut Pro generators typically do not extend past 2 minutes once in the timeline. Often editors duplicate the generators in the timline to obtain the desired length. There is a easy way to be able to extend generators. When a generator is first opened in the viewer the default in and out points are set 10 seconds apart, but if you change the value in the  duration window you can extend your clip out as far as the value entered in the duration window. If you already have brought a generator into the timeline, you can double click on it, load it into the viewer, and make the change to the duration.