There is a lot of subjectivity and a wide range of information online about what constitutes an "active" starter.
In Trevor J Wilson's Book, Open Crumb Mastery, p. 238, he states that beginners often misjudge their starters to be active. According to Trevor, the rate of rise is the most telling factor. He goes on to describe what he considers workable times and volumes. (Pick up a copy of his e-book at www.breadwerx.com. It's a fabulous read.)
I've decided to put on my old marketing research hat and create a survey about starters. I think it would be very interesting to survey a large volume of bakers and see how active the yeast is. Most of us pick up nuggets here and there on the internet or in a book about how much our starters should grow in x amount of time. I know, I know, there are many MANY factors here, everything is relative, and there is only so much we can quantify. However, that never stops us from trying and if we get enough responses to this survey, the data set as a whole can start to give us a broad picture about what active means.
Anybody who takes the survey will have full access to the answers for as long as the survey is up and running. (I really hope the baking veterans here will participate. We need your responses!)
Here is the link to the survey. (It's a google form so you can feel safe about clicking on it.)
Be sure to bookmark/save the link with the spreadsheet results after you've taken the survey. That way, you can return to it over and over again and see new responses as they come in.
P.S. I have very specifically asked questions in a certain way and in a certain order. When doing this type of research, we're always striving to balance a lot of things -- succinctness vs. specification, length vs. detail, etc. There are many other areas I could have probed and didn't. That being said, if you find anything confusing or gaping holes in the questions, do let me know. I have no pride of authorship and surveys are always biased by the survey writer to some extent.