Seeing the time on page metric for email traffic sources, aims to inform how well email is driving traffic to a webpage. However due to flaws in the time on page methodology it can be quite inaccurate.
Time on page measures the difference between when opens a webpage and then clicks another link. Or opens a new tab and visits the same domain. A common fallacy is that time on page represents all users, when it only captures those that complete those actions. So it is not reflective of an overall audience. It tends to skew up.
It would be better to use a newer metric like Attention, which measures second by second, how long someone engages on a page for ALL users. This gives a more accurate view of what the customer is doing.