Hi, so i have somes scenes on my blender file and they all have different duration.
For instance, if I have 2 scenes, one with 100 keyframes and the other with just 50, I have compositing output working for both, BUT Blender will render unnecessary keyframes if I press Ctrl+F12 on the first scene… (AND RENDERS FRAMES FROM 51 TO 100 ON THE SECOND!)
Is there some way to renders multiples scenes with each scene duration preserved?
Thank you
In the compositor, the only duration that matters is the one for the scene you are in when you press Ctrl + f12. Here I will explain with a story:
Let’s say Scene_A goes from frame 1 to frame 250, and Scene_B goes from frame 20 to frame 50. In the compositor, Scene_A uses a render layer from Scene_B.
You are in Scene_A and you hit ctrl + f12. Now we are rendering everything on frame 1.
“But I’m not supposed to start yet,” says Scene_B. “I only go from frame 20 to 50!” But it doesn’t matter. We are rendering frame 1 and we will keep rendering all the way to 250!
If you want to render multiple scenes with duration preserved, use the Video Sequencer. You cannot use it for advanced compositing but you can control the timing of every scene.
I would always advise to use comand line rendering. More information about this can be found in the Blender manual: https://docs.blender.org/manual/en/latest/advanced/command_line/render.html. You can then write a batch file with your commands and everything will be rendered in sequence - even faster than using the render command in the blender user interface 
. An example for your case would be something like this:
cd C:\Program Files\blender\blender-2.82
blender -b "Path To Blend File" -S "Scene 1 Name" -s 1 -e 50 -a
blender -b "Path To Blend File" -S "Scene 2 Name" -s 51 -e 100 -a
Thanks @Rocketman! This is what I was looking for!
Video sequencer solved my problems!
Thanks for the story! 