I do not know what you mean by the page being "live".
The Adobe time spent is calculated by looking at the time stamp of a hit, then subtracting it from the time stamp of the next hit. A hit can be a page view type call or a custom link type call. It includes the instance of a value and persistence (for evars) across all subsequent hits. In the case of props, time spent is counted across subsequent custom link type calls.
If your page is the last page in the visit, or if your visit has only a single call (e.g. a single page visit) the time spent will not be counted for that past PV.

In the example above, the Total Seconds Spent for the whole visit is 420 seconds (7 minutes). The time spent on the Pill Identifier page is 0, as this was the last call in the visit (the exit page). The time spent on the Gerd page is 240 seconds (4 minutes), which includes the three custom link type calls.
From that perspective, your 15 second custom call might catch that last page or the page in a single page call visit.
However, that can be an expensive proposition as you are paying for all those 15 second calls. We did an analysis of the difference between trying to send a call every 10 seconds ourselves compared to Adobe's default method. The difference was so small as to not even be a consideration in decision making.