With 80 experience for each dust every third second the experience rate of playing the harp is a maximum of 96,000 per hour in Crafting. This can however be boosted with artisan's outfit to boost the experience rates to just over 100,000 experience per hour. With tuning and a possibility to not get any dust the actual experience gain each hour is considerably lower (around 60,000 per hour at level 90). You also get 50 experience in Construction for each 10% the harp is out of tune. Note that the harp will give significantly less dust when it is 50% or more out of tune. When the Voice of Seren is illuminating the Ithell district each dust gives 96 experience and tuning the harp gives 60 experience per 10% out of tune. While the Voice is active, harps also go out of tune at a much lower rate, meaning you will almost always gain more harmonic dust per hour while it is active.