InstancedMesh Overheads Cost

I have a question on instanced static mesh component.

I’m sure at a certain level there are some overheads in using InstancedMesh.
For this I’m wondering, how are the overheads like in doing InstancedMesh?
In general. how many instance do you have to generate to make it worth instancing? ( able to cover the cost of instancing ) . Does polygon matter in these overhead or just number of instances.