My experience on PC 8801 emulator, it costs my 40 hours more in first time to play (And I cannot beat it cause dragon’s damage too high)
5 to 6 hours I think it’s hard for a player, unless you know all the tricks about this game (without guide book you probaly need to paid 100+ hours on discover all the things). But I heard dragon slayer on Gameboy is much harder than PC8801, so I have no idea, maybe I’ll tried it later.