Just by sunning the Dried Shiitake you purchased can increase the Vitamin D content by 24 times. The average dried Shiitake (includes both artificially and naturally grown) contains 12.7μg of Vitamin D2 (Japanese Ministry of Education Data). Naturally grown Dried Shiitake contains 87μg or 7 times the average Dried Shiitake. Recent studies show Vitamin D2 of Shiitake has the same effect as Vitamin D3 Supplements. If you place the Dried Shiitake under a strong U V Index greater than 6 for one hour is enough to increase the Vitamin D2 content by 24 times.
(The U.S. National Center of Biology Institute (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3897585/)
A Kobe Women’s University of Pharmacy study has documented that under 3 hours of sunshine at 14.6mW/cm2 (using 1 Dried Shiitake having Vitamin D2 contents of 20IU):
After 1 hour exposed to sunshine increased to 488IU (24x)
After 3 hours exposed to Sunshine increased to 548 IU (27x)
The difference between one and two hours does not appear significant so I believe one hour is enough.
For your information, Sugimoto Shiitake by drying our Shiitake using deep infrared, not only reduces the moisture content to 9% but also raises the average Vitamin D content to 200 μg or 8,000 IU or 15 times the average 12.7μg Shiitake.