You may want to use the content engagement score that is provided by Marketo. It combines open rates, click rates and unsubscribe rates.
From my experience, it is not sure that you will observe a decline. It really depends on the quality of the content. If the content in good, it might even be the opposite...
Thanks, I'm not sure I will see decline either although our current nuture campaign at the moment is pretty basic and not great. I'm just hoping to see what happens now to benchmark against my improvements.
I can only see engagement score against the emails themselves not against time someone has spent in the nuture campaign or the amount of emails they have received.