How to stop rendering a renderlayer at any moment of the animation? I have 3 render layers that I'm compositing together in the 4th one. Everything works fine, but even if I don't see the layer in some specific moment blender still renders it. The problem is that I use the Cycles volumetric light for a short time then it disappears and we don't see it anymore but it still takes forever to render even though every single light in the layer fades to black. How can I make it stop rendering what I don't need anymore?

P.S. I know, I can render it part by part and then composite a video, I can even render it with image sequence to not to lose any quality with re-encoding video, but it would be wonderful if it could be done withing blender compositor and animation system.
Renderlayers have a checkbox next to them which controls whether or not the renderlayer is rendered. 

This checkbox can be animated (like most options in blender) by pressing I while hovering over the check box or by pressing  RMB> Insert keyframe to insert a keyframe.

Note that this will make the renderlayer node not output an image, so your compositing setup must be carefully designed to account for that.

If the layer you want to toggle has no alpha transparency, a simple alpha over node will let you put another image in place of the renderlayer when it is not enabled (in this example, I used a mask node to provide a solid color the same size as the render):



If the layer does have alpha transparency, this won't work.

Instead you'll have to use an animated mix setup or something.