Inserting rotation keyframes First: Thanks to R-800 for his/her quick answer. Though, it wasn't really what I looked for.

I try to be more specific about my former question:
I have an animation of 1200 frames. My objects perform some motion until frame 200. From frame 200 to 360 my objects rotate about the z-axis completing 2 full cycles and then move further. 
I created the rotating motion by inserting key frames every 10th frame rotating the objects 45 degrees further. Thus at frame 360 my objects have a rotation angle about the z-axis of 720 degrees.

I now want them to rotate not only 2 cycles but 4 from f200 to f520. I.e. I want to insert another 160 rotation frames after frame 360.

I tried to shift all the original key frames after f360 to start at f520 and then fill the gap again with 45 degree rotations. This worked well until frame 520 then the motion got screwed up because the rotation angle at f520 was 4*360=1440 degrees but key frame 530 (the shifted one originally at f370) had an angle of 720 degrees and the objects rotate all the way back from 1440 deg to 720 deg within the 10 frames from f521 to f530.

That's where I'm stuck now. Of course I could set all the following keyframes after f520 to the correct rotation angle. But I thought there must be an easier way.
You can create cycles instantly using Curve modifiers, which are accessed through the Graph Editor window's Properties Shelf (press 'n' while mouse hovering over the Graph Editor Window to bring up the Shelf). Once the Properties Shelf is up, you will see tabs to the far right of it, one of which is Modifiers. Click on it.



In my example, I created two keyframes on the Global Z axis to make the cylinder rotate 360 degrees in 32 frames. Instead of making extra keyframes to add additional revolutions, I was able to create a cycle modifier, which automatically extends the rotations indefinitely. Setting the Repeat Motion: After field to 3 allows me to add three repeated revolutions to the original one, giving me four revolutions, total.

You may find, as I did, that something odd happens to the final keyframe in the original F-Curve after adding the Repeat Motion restriction, causing the cycled loops to have timing which does not connect seamlessly. I had to manually adjust the handles on the final keyframe for the original F-Curve after setting this up, for reasons unknown. Might be a bug. Just giving you a heads up.