How do I limit frame range for imported Blender actions?

I have created a model in Blender with some actions. Let’s say an idle animation and a walk one.
My walk animation has keyframes setted like this:

|-10   |0     |10    |20    |30    |

When I import my .fbx file actions are imported correctly but I want walk animation to play from frame 0 to frame 20, not from the very first keyframe (-8 in the example) to the very last one (35 in the example).
In Blender I used keyframes exceeding the animation range to achieve a good loop so I would not delete them.
How can I do this?

Why not just set the start and end frame of the animation inside Blender? You can still have the key frames beyond the start and end.

In your example, when editing the animation in Blender, set the start frame to 0, end to 20. Unity will understand the animation is between the start and end frames you have set in Blender, and will not use the frames -8 to 0 and 21 to 35.

Maybe this, this and this could help too.