If you do a dungeon run with competent players, it will at most take you 20mins, but with new players in DR, it can take anywhere from 20-35 average with more time if people are not paying attention to mechanics or doing enough dps. 40 extra tomes of law is pointless and not enough compensation to deal with this.

If SE wants to encourage more people to help other new players, the bonus needs to be at least double, both XP and tomes. Then it would mean you can spend 40 minutes inside ONE dungeon and still get the equivalent of 2 dungeons.