Hi Shea,
Unfortunately there's not a clear way to measure how much "active time" was spent filling out a form. Even without the Incomplete Forms mechanism, the app would need a way to identify actions like putting the form in the background and resuming it (something that's fairly common in some workflows).
Long term, we do have plans to build out a system to report more granular metrics about form sessions which could be analysed to capture this kind of data, but due to the complexities mentioned I think it's actually going to need to be a pretty big lift.
In the mean time if you search the forum history, there are some examples of various tricks for doing things like applying timestamps to individual questions or sections of forms. You could give them a shot to see if any of them would help give you a bit more visibility, but I'm not sure how well they'll intersect with incomplete forms.