You must now composite these in a video editor. You then set frame rate and output type (most choose mp4) More advanced editors have many more options, frame blending for example. I would also check the Terragen Animation page here in the forum, there are some useful links there.
There are many free or trial packages available. I think Photoshop can do this also. I use Blender 2.79 which is free. Microsoft has a video editor, but it is more for slide shows in my opinion for the home user.
For 350 frames, you will probably yield animation 8 to 10 seconds, depending....