This is an awesome feature. But to be more useable, we should be able to use it as soon as it is released. Which is not possible today. Because we have to set a date and then wait for that date to pass. For it to be useful to use today, this feature needs to be able to look at historical data. For example, I'd like to see a list of articles that haven't been modified for the last one month. This information can be easily collated from the article timestamps. There exists today a set of articles that meet the set criteria - as of today. I shouldn't have to wait from today all over again.
If we can pull out a report today, and not have to wait for 15, 30 days as is how it is designed, it would be much more useful.
Also, it's not clear about what the current functionality does. Let's say I set the initial stale date at 15 days. I wait 15 days and then generate the report, or look at all the articles that have become stale. Now let's say this list is too much and I choose to increase the period to 30 days. Now, will I have to wait another 30 days - so all in all 45 days from the time the feature was made available.