I am creating choropleth maps for quantified data. Each DDP will focus on a new county, which is shown by using a clip of the data frame (by the county boundary). I want to define quantified legend symbology by equal intervals, which will change for each DDP. This means that the legend's intervals would change for each page. For example, Montgomery county may have a data range of 0-20, therefore the legend's intervals would be 0-10, 10-20, 20-30, and 30-40. Bucks County may have a data range of 0-400, therefore the legend's intervals would be 0-100, 100-200, 200-300, and 300-400.
Unfortunately I don't believe there is a way in ArcGIS to create a dynamic legend with what you are looking for. The dynamic legend won't be able to change intervals with each county, only if you add each county as a separate layer. Or, like someone else has said, changing the interval scales overall.
I have read about ways to create more personalized dynamic features using python script, so that could be an area to look into.