"time spent" is not a metric available to us mortals, maybe (well, hopefully!) not even to the devs. I guess the best you can do from the data dumps is to extract
- The frequency of answers
- The frequency of comments
- The frequency of edits
and interpolate some sort of activity index from them. Of course that is very hazy, because you'll have to guess how long a user needs on average to write an answer, for example ... Links could increase that time (because presumably, the user needs to copy&paste them from somewhere); edits over a short time span could drive the index up as well.
It's not much, but I think it's the best you can hope to achieve with the data available.