on the document properties panel reduce the FPS ( frames persecond ) the less frames per second the longer the animation,
and the F6 keyframe just adds another keyframe which would work i guess, give it a go by pressing right click insert keyframe, but all that will do is extend the last keyframe unless you add to it...
what do you want to achieve by making the length longer?
Just count out however many frames (you say 120), right click that frame in the desired layer and select the command to Insert Frame. That will extend the timeline to that frame without adding keyframes.
Not sure why internet said F6, as I dont want keyframes, simply the fact that I had created an annotated slide show with text placed in specific locations, fading in and out whilst relating to features on the slides, but then I realised the duration of the captions was too quick to allow reading of them, so to extend them I also needed to extend the slides they related to, keeping the relationship between slides the same, so an extra e.g 5 secs reading time and slide viewing time for the slides is required.
Thanks for the F5 tip. Highlight e.g. 1 secs worth of frames and hit F5 5 times to add in 5 secs and extend everything aft of this. I didnt have 5 secs worth of frames to highlight, so did it this way.
I had the entire slide show across one timeline, no movieclips, as I could then see timeline wise how the captions faded out as the next slide faded in. I could play with the fade between slide transitions this way.