will last a very long time if properly maintained. Maintance means keeping the bad ones replaced with new, keeping a good penetrating paint/stain like Woodscapes on them. Shakes do curl and to twist and contourt, but that is their nature. Keeping them maintained will make them last.
Now where you have to be careful is when you apply an extension ladder up there to check them out. this will damage the shakes. Consider attaching a stand off to your ladder and then wrap the legs with a heavy cotton towel to make the legs of the stand off more friendly to the shakes.
Then too, painting the shakes usually always requires the use of a spray rig. Hand painting is possible but will take a long time.