Can you talk a bit more about your scenario? Are you wanting Target to respond back after a certain time on page or are you wanting to create an audience of people who spend more than X time on Y page? Time on page is more of an analytics thing, but you could certainly track that client side and create a profile variable for the active user and what their average time on page is.
Yes, I would like to know, how to retarget the user on the basis of time.
Suppose, there are two pages A and B, if a user spends 5 min on page A and the same user spends 10 min on page B then page B timings > page A hence the content will be displayed related to page B on the homepage or any retargeting page.