I shall now ruin every single one of you by being pedantic.
Hell in Gensokyo is implied to be based off the Buddhist idea of Naraka. Physically, the Narakas are described as being a series of layers below the Earth's surface. The things that lie within these caverns vary from source to source, although the most prevalent ones are the Eight Cold Narakas and Eight Hot Narakas.
One example is the Arbuda ("blister") Naraka, where the length of life in it is said to be equivalent to the time one would take to empty a barrel of sesame seeds if one removes only one seed every century. Assuming the barrel's volume is one cubic metre (100,000 cubic centimetres), and the sesame seed's volume is 0.00008 cubic cm (based on my rough estimations), one sentence in the Arbuda Naraka would take 1250000000 years, which is 1250 million years.
Say, as you have asked, if Shikieiki sentences a sinful soul to one million years in Hell, the offence committed by said sinner would be approximately 1250 times less severe than an offence that would land him or her in the Arbuda Naraka.
To put it simply, don't anger the Yamas.
Edit: Was busy typing a long answer, wound up getting ninja-d:
For LCS' question: It would be a at the summit of Youkai Mountain.
For Hannibal: It would heavily involve Raiko because of the word "Thunder" in the title.
What if everybody on this thread gave long-winded, pedantic answers?
