I am trying to intersect 420k lines with 25 lines, but the intersect tool takes forever (over 17 hours). I wonder if it will succeed doing so and I really think ArcGIS is actually stalling. Is there a way to make sure ArcGIS is still running correctly? And why is it taking so long in the first place?
Adding to dafiter's comment, you could also turn off the background processing. This will ensure you know the progress of the geoprocessing instance, and also is faster than that with Background Processing enabled.
Foreground and background processing