Denoiser wastes quite a time on empty renderlayer (about 2 mins for 1080p), but its checkbox can't be keyframed and even LMB's Copy data path is greyed out (so, no scripting I guess).
Animating renderlayer visibility is no option for me, because it messes Multilayer EXR a lot.
Ended up with this code, where denoiser starts working on specific renderlayer only on certain frames.
import bpy
scene = bpy.context.scene
def denoiser_control (scene):
if scene.frame_current in list(range(66, 90)) + [1, 20, 50]:
bpy.data.scenes["Scene"].render.layers["RenderLayer"].cycles.use_denoising = True
else:
bpy.data.scenes["Scene"].render.layers["RenderLayer"].cycles.use_denoising = False
bpy.app.handlers.frame_change_pre.append(denoiser_control)
range(66, 90) stands for some frame range and [1, 20, 50] defines single frames.