So, in the store description for Elvira: Mistress of the Dark it reads "Over 100 hours of frightening gameplay". Is this accurate? Because it seems a bit much for this kind of game. Where do they get the number?

Very large exageration lol. If you go in blind and dont use a guide, maybe 20-30 hours. But as with all these old games it really depends how good you are at solving the puzzles. If you use a guide and casually play you can finish it in 2 hours. The combat and puzzles arent particulary complicated, just save often and dont use spells unless you really have to and keep track of your items as there are softlocks. Refer to the spellbook outside the game as this is the games copy protection.